/* OnthiDH – bản web chạy thử. Ưu tiên điện thoại, khung tối đa 720px. */
:root {
  --mau: #2F6FED;
  --nen: #F5F6FA;
  --mat: #FFFFFF;
  --mat-2: #EEF0F5;
  --chu: #1B1F2A;
  --chu-mo: #5B6272;
  --vien: #DCE0E8;
  --dung: #12965A;
  --dung-nen: #E3F6EC;
  --sai: #D63B3B;
  --sai-nen: #FDE9E9;
  --vang: #B7791F;
  --vang-nen: #FFF4DB;
  --bong: 0 1px 2px rgba(20, 24, 40, .06), 0 2px 8px rgba(20, 24, 40, .05);
  --bo: 14px;
  --mau-nhat: color-mix(in srgb, var(--mau) 12%, var(--mat));
  --mau-dam: color-mix(in srgb, var(--mau) 82%, #000);
  color-scheme: light;
}
@media (prefers-color-scheme: dark) {
  :root {
    --nen: #0F1218;
    --mat: #181C24;
    --mat-2: #222733;
    --chu: #E8EAF0;
    --chu-mo: #9AA2B4;
    --vien: #2E3442;
    --dung: #3CCB85;
    --dung-nen: #133826;
    --sai: #FF6B6B;
    --sai-nen: #3D1B1E;
    --vang: #F2B544;
    --vang-nen: #3A2E12;
    --bong: 0 1px 2px rgba(0, 0, 0, .4);
    --mau-nhat: color-mix(in srgb, var(--mau) 22%, var(--mat));
    --mau-dam: color-mix(in srgb, var(--mau) 70%, #fff);
    color-scheme: dark;
  }
  .hinh img { background: #fff; border-radius: 8px; padding: 4px; }
}

* { box-sizing: border-box; }
html { -webkit-text-size-adjust: 100%; }
body {
  margin: 0;
  background: var(--nen);
  color: var(--chu);
  font: 16px/1.6 "Be Vietnam Pro", system-ui, -apple-system, "Segoe UI", Roboto, "Helvetica Neue", Arial, sans-serif;
  overflow-x: hidden;
  -webkit-tap-highlight-color: transparent;
}
a { color: var(--mau-dam); }
h1, h2, h3 { line-height: 1.3; }
h2 { font-size: 1.15rem; margin: 0 0 .6rem; }
button, input { font: inherit; color: inherit; }
img { max-width: 100%; height: auto; }
:focus-visible { outline: 3px solid color-mix(in srgb, var(--mau) 55%, transparent); outline-offset: 2px; }

.khung { max-width: 720px; margin: 0 auto; padding: 12px 16px 48px; }
.mo { color: var(--chu-mo); font-size: .92rem; }
.lon { font-size: 1.1rem; font-weight: 600; }
.chu-thich { color: var(--chu-mo); font-size: .85rem; text-align: center; margin-top: 24px; }
.muc { font-size: 1.05rem; margin: 22px 0 10px; }

/* ---------- Thanh trên ---------- */
.thanh-tren, .thanh-de {
  position: sticky; top: 0; z-index: 20;
  display: flex; align-items: center; gap: 10px;
  padding: 8px max(12px, calc((100vw - 720px) / 2 + 12px));
  padding-top: max(8px, env(safe-area-inset-top));
  background: color-mix(in srgb, var(--mat) 92%, transparent);
  backdrop-filter: saturate(1.4) blur(8px);
  border-bottom: 1px solid var(--vien);
  min-height: 56px;
}
.tt-tieu-de { flex: 1; font-size: 1.05rem; margin: 0; white-space: nowrap; overflow: hidden; text-overflow: ellipsis; }
.nut-ve {
  display: inline-grid; place-items: center; width: 44px; height: 44px; margin-left: -8px;
  font-size: 2rem; line-height: 1; text-decoration: none; color: var(--chu); border-radius: 12px;
}
.nut-ve:hover { background: var(--mat-2); }
.logo { display: inline-grid; place-items: center; }

/* ---------- Nút ---------- */
.nut {
  display: inline-flex; align-items: center; justify-content: center; gap: 6px;
  min-height: 44px; padding: 8px 16px; border-radius: 12px;
  border: 1.5px solid var(--vien); background: var(--mat); color: var(--chu);
  font-weight: 600; text-decoration: none; cursor: pointer; white-space: nowrap;
  transition: transform .06s, background-color .15s;
}
.nut:active:not(:disabled) { transform: translateY(1px); }
.nut.chinh { background: var(--mau); border-color: var(--mau); color: #fff; box-shadow: 0 2px 0 var(--mau-dam); }
.nut.chinh:hover { filter: brightness(1.06); }
.nut:disabled { opacity: .45; cursor: not-allowed; box-shadow: none; }
.nut.nho { min-height: 40px; padding: 6px 12px; font-size: .92rem; }
.nut.rong { width: 100%; }
.lon-nut { width: 100%; min-height: 52px; font-size: 1.05rem; }
.hang-nut { display: flex; flex-wrap: wrap; gap: 10px; align-items: center; margin-top: 14px; }
.hang-nut.cuoi { justify-content: flex-end; margin-top: 20px; }
.dem { display: inline-grid; place-items: center; min-width: 20px; height: 20px; padding: 0 5px; border-radius: 10px; background: var(--sai); color: #fff; font-size: .75rem; }

/* ---------- Thẻ chung ---------- */
.the { background: var(--mat); border: 1px solid var(--vien); border-radius: var(--bo); padding: 16px; margin: 12px 0; box-shadow: var(--bong); }
.trong-rong { text-align: center; padding: 28px 16px; }
.chip {
  display: inline-flex; align-items: center; padding: 2px 8px; border-radius: 999px;
  background: var(--mat-2); color: var(--chu-mo); font-size: .78rem; font-weight: 500; white-space: nowrap;
}
.chip.muc { background: var(--mau-nhat); color: var(--mau-dam); }

/* ---------- Trang chủ ---------- */
.chao { display: flex; align-items: center; gap: 14px; margin: 8px 0 4px; }
.chuoi {
  display: flex; flex-direction: column; align-items: center; justify-content: center;
  min-width: 84px; padding: 10px 8px; border-radius: 16px; background: var(--mat); border: 1px solid var(--vien);
  font-size: .72rem; color: var(--chu-mo); line-height: 1.2; text-align: center;
}
.chuoi b { font-size: 1.6rem; color: var(--chu); }
.chuoi .lua { font-size: 1.3rem; filter: grayscale(1); opacity: .5; }
.chuoi.bat .lua { filter: none; opacity: 1; }
.chuoi.bat { border-color: #FFB020; background: color-mix(in srgb, #FFB020 12%, var(--mat)); }
.chao-loi { margin: 0; font-weight: 500; }
.hoc-tiep {
  display: grid; grid-template-columns: 1fr auto; align-items: center; gap: 2px 10px;
  margin: 14px 0 4px; padding: 14px 16px; border-radius: var(--bo);
  background: var(--mau); color: #fff; text-decoration: none; box-shadow: 0 3px 0 var(--mau-dam);
}
.ht-nhan { font-size: .8rem; opacity: .9; text-transform: uppercase; letter-spacing: .04em; }
.ht-ten { grid-column: 1; font-weight: 600; }
.ht-mui { grid-row: 1 / 3; grid-column: 2; font-size: 1.8rem; }
.ds-mon { display: grid; gap: 12px; grid-template-columns: repeat(auto-fill, minmax(200px, 1fr)); }
.the-mon {
  --mau-mon: var(--mau);
  display: flex; flex-direction: column; gap: 4px; padding: 18px 16px 14px; min-height: 140px;
  border-radius: 18px; text-decoration: none; color: var(--chu);
  background: color-mix(in srgb, var(--mau-mon) 10%, var(--mat));
  border: 2px solid color-mix(in srgb, var(--mau-mon) 35%, transparent);
  box-shadow: 0 3px 0 color-mix(in srgb, var(--mau-mon) 35%, transparent);
  transition: transform .08s;
}
.the-mon:active { transform: translateY(2px); box-shadow: 0 1px 0 color-mix(in srgb, var(--mau-mon) 35%, transparent); }
.tm-ten { font-size: 1.35rem; font-weight: 700; color: color-mix(in srgb, var(--mau-mon) 75%, var(--chu)); }
.tm-phu { font-size: .88rem; color: var(--chu-mo); }
.tm-tien-do { margin-top: auto; height: 8px; border-radius: 4px; background: color-mix(in srgb, var(--mau-mon) 15%, var(--mat-2)); overflow: hidden; }
.tm-tien-do span { display: block; height: 100%; background: var(--mau-mon); border-radius: 4px; }
.tm-so { font-size: .8rem; color: var(--chu-mo); }

/* ---------- Hộp thoại ---------- */
.hop-nen { position: fixed; inset: 0; z-index: 100; background: rgba(10, 12, 20, .45); display: grid; place-items: center; padding: 16px; animation: hien .12s ease-out; }
.hop { width: min(420px, 100%); background: var(--mat); border-radius: 18px; padding: 20px; box-shadow: 0 10px 40px rgba(0, 0, 0, .25); }
.hop h2 { margin-top: 0; }
.hop-nut { display: flex; justify-content: flex-end; gap: 10px; margin-top: 16px; }
.hop-nut .nut { flex: 1; }
.hoac { text-align: center; color: var(--chu-mo); font-size: .85rem; margin: 14px 0 8px; }
.dn-email { display: flex; gap: 8px; }
.dn-email input { flex: 1; min-width: 0; min-height: 44px; padding: 8px 12px; border: 1.5px solid var(--vien); border-radius: 12px; background: var(--mat); }
@keyframes hien { from { opacity: 0; } }
.thong-bao {
  position: fixed; left: 50%; bottom: max(24px, env(safe-area-inset-bottom)); transform: translateX(-50%); z-index: 120;
  background: var(--chu); color: var(--nen); padding: 10px 18px; border-radius: 12px; font-weight: 500; max-width: calc(100% - 32px);
  animation: hien .15s ease-out;
}
.thong-bao.ok { background: var(--dung); color: #fff; }

/* ---------- Trang môn ---------- */
.tabs { display: flex; gap: 4px; padding: 4px; background: var(--mat-2); border-radius: 14px; margin: 4px 0 12px; }
.tabs a {
  flex: 1; text-align: center; padding: 10px 6px; min-height: 44px; border-radius: 11px;
  text-decoration: none; color: var(--chu-mo); font-weight: 600; font-size: .95rem;
}
.tabs a[aria-selected="true"] { background: var(--mat); color: var(--mau-dam); box-shadow: var(--bong); }
.nhom-cd h3 { font-size: .85rem; text-transform: uppercase; letter-spacing: .04em; color: var(--chu-mo); margin: 20px 4px 8px; }
.o-cd {
  display: flex; align-items: center; gap: 12px; padding: 12px 14px; min-height: 60px; margin-bottom: 8px;
  background: var(--mat); border: 1px solid var(--vien); border-radius: var(--bo); text-decoration: none; color: var(--chu);
}
a.o-cd:hover { border-color: var(--mau); }
.ocd-ma {
  flex: none; display: grid; place-items: center; width: 40px; height: 40px; border-radius: 12px;
  background: var(--mau); color: #fff; font-weight: 700;
}
.ocd-ten { flex: 1; min-width: 0; font-weight: 500; line-height: 1.35; }
.ocd-tien-do { display: block; height: 6px; margin-top: 6px; border-radius: 3px; background: var(--mat-2); overflow: hidden; }
.ocd-tien-do span { display: block; height: 100%; background: var(--mau); }
.ocd-pt { font-size: .85rem; color: var(--chu-mo); font-weight: 600; }
.o-cd.sap-co { opacity: .6; }
.o-cd.sap-co .ocd-ma { background: var(--mat-2); color: var(--chu-mo); }
.nhan-sap { font-size: .78rem; color: var(--chu-mo); background: var(--mat-2); padding: 2px 8px; border-radius: 999px; white-space: nowrap; }
.nhan-duyet { font-size: .7rem; color: var(--vang); background: var(--vang-nen); padding: 1px 6px; border-radius: 999px; vertical-align: 1px; }
.huy-hieu { display: inline-flex; align-items: center; gap: 4px; color: #B7791F; font-weight: 700; font-size: 1.3rem; }
.huy-hieu.lon { font-size: .9rem; background: #FFF1C9; color: #8A5A00; padding: 3px 10px; border-radius: 999px; }
.ds-de { display: grid; gap: 10px; grid-template-columns: repeat(auto-fill, minmax(150px, 1fr)); }
.o-de {
  display: grid; grid-template-columns: 1fr auto; gap: 4px; align-content: start; padding: 14px; min-height: 104px;
  background: var(--mat); border: 1px solid var(--vien); border-radius: var(--bo); text-decoration: none; color: var(--chu);
}
.o-de:hover { border-color: var(--mau); }
.od-so { font-weight: 700; font-size: 1.1rem; }
.od-phu, .od-diem { grid-column: 1 / -1; font-size: .85rem; color: var(--chu-mo); }
.od-diem b { color: var(--mau-dam); font-size: 1rem; }
.nhan-pro { align-self: start; font-size: .72rem; font-weight: 700; color: #8A5A00; background: #FFF1C9; padding: 2px 7px; border-radius: 999px; white-space: nowrap; }
.nhan-free { align-self: start; font-size: .72rem; font-weight: 600; color: var(--dung); background: var(--dung-nen); padding: 2px 7px; border-radius: 999px; white-space: nowrap; }
.dang-lam { color: var(--vang); font-weight: 600; }

/* ---------- Làm đề ---------- */
.thanh-de .td-giua { flex: 1; display: flex; flex-direction: column; align-items: center; line-height: 1.15; min-width: 0; }
.dong-ho { font-variant-numeric: tabular-nums; font-weight: 700; font-size: 1.15rem; }
.dong-ho.sap-het { color: var(--sai); }
.td-tien-do { font-size: .78rem; color: var(--chu-mo); }
.ngan-bang {
  position: sticky; top: 56px; z-index: 19; max-height: 55vh; overflow: auto;
  background: var(--mat); border-bottom: 1px solid var(--vien); box-shadow: 0 8px 20px rgba(0, 0, 0, .08);
  padding: 12px max(16px, calc((100vw - 720px) / 2 + 16px));
}
.bang-cau { display: grid; grid-template-columns: repeat(auto-fill, minmax(44px, 1fr)); gap: 6px; }
.o-so {
  display: inline-grid; place-items: center; min-height: 44px; border-radius: 10px; border: 1.5px solid var(--vien);
  background: var(--mat); font-weight: 600; cursor: pointer; position: relative; font-style: normal;
}
.o-so.da { background: var(--mau); border-color: var(--mau); color: #fff; }
.o-so.mot-phan { background: var(--mau-nhat); border-color: var(--mau); }
.o-so.co::after { content: ""; position: absolute; top: 3px; right: 3px; width: 8px; height: 8px; border-radius: 50%; background: #F59E0B; }
.chu-giai { display: flex; flex-wrap: wrap; gap: 6px 14px; margin-top: 10px; font-size: .8rem; color: var(--chu-mo); }
.chu-giai span { display: inline-flex; align-items: center; gap: 6px; }
.chu-giai .o-so { min-height: 16px; width: 16px; height: 16px; border-radius: 4px; cursor: default; }
.chu-giai .o-so.co::after { top: 3px; right: 3px; width: 6px; height: 6px; }
.ten-de { font-size: 1.1rem; margin: 8px 0 4px; }
.phan { margin-top: 18px; }
.phan-dau {
  display: flex; flex-wrap: wrap; align-items: baseline; gap: 4px 10px; padding: 10px 14px; margin: 0 0 10px;
  border-radius: 12px; background: var(--mau-nhat); border-left: 4px solid var(--mau);
}
.phan-so { font-weight: 700; color: var(--mau-dam); }
.phan-ten { font-weight: 600; }
.phan-phu { font-size: .85rem; color: var(--chu-mo); }
.cuoi-de { margin: 28px 0 8px; }

/* ---------- Câu hỏi ---------- */
.cau {
  background: var(--mat); border: 1px solid var(--vien); border-radius: var(--bo); padding: 14px 14px 16px; margin: 0 0 12px;
  scroll-margin-top: 72px; font-size: 1.0625rem; transition: box-shadow .3s;
}
.cau.nhay { box-shadow: 0 0 0 3px color-mix(in srgb, var(--mau) 45%, transparent); }
.cau-dau { display: flex; flex-wrap: wrap; align-items: center; gap: 6px; margin-bottom: 8px; }
.cau-so { font-weight: 700; color: var(--mau-dam); margin-right: 2px; }
.nut-co {
  margin-left: auto; width: 44px; height: 44px; border-radius: 10px; border: 1.5px solid transparent; background: transparent;
  color: var(--chu-mo); font-size: 1.15rem; cursor: pointer;
}
.nut-co.bat { color: #D97706; background: #FEF3C7; border-color: #F59E0B; }
.cau-dan { overflow-wrap: anywhere; }
.vb > :first-child { margin-top: 0; }
.vb > :last-child { margin-bottom: 0; }
.vb p { margin: .5em 0; }
.vb ul, .vb ol { padding-left: 1.3em; margin: .5em 0; }
.vb blockquote { margin: .5em 0; padding: 4px 12px; border-left: 3px solid var(--vien); color: var(--chu-mo); }
.khung-vb { margin: 6px 0; }
.khung-vb.co-khung { border: 2px solid var(--mau); border-radius: 12px; padding: 12px 14px; background: color-mix(in srgb, var(--mau) 4%, var(--mat)); }
.khung-tieu-de { font-weight: 700; text-align: center; margin-bottom: 6px; font-size: 1.05em; }
.cuon { overflow-x: auto; -webkit-overflow-scrolling: touch; max-width: 100%; }
table { border-collapse: collapse; margin: 8px 0; font-size: .95em; }
th, td { border: 1px solid var(--vien); padding: 6px 10px; text-align: center; vertical-align: middle; }
th { background: var(--mat-2); font-weight: 600; }
.hinh { margin: 10px auto; text-align: center; max-width: 100%; }
.hinh img { max-width: 100%; }
.hinh figcaption { font-size: .82rem; color: var(--chu-mo); margin-top: 4px; }
.cong-thuc { text-align: center; margin: 8px 0; }
mjx-container { max-width: 100%; }
mjx-container[display="true"] { overflow-x: auto; overflow-y: hidden; padding: 2px 0; }

.pa-ds { display: grid; gap: 8px; margin-top: 12px; }
.pa-ds.luoi { grid-template-columns: 1fr 1fr; }
.pa {
  display: flex; align-items: center; gap: 10px; width: 100%; min-height: 48px; padding: 8px 12px;
  text-align: left; border: 2px solid var(--vien); border-radius: 12px; background: var(--mat); cursor: pointer;
  box-shadow: 0 2px 0 var(--vien); transition: border-color .12s, background-color .12s;
}
.pa:hover:not(:disabled) { border-color: color-mix(in srgb, var(--mau) 50%, var(--vien)); }
.pa .nhan {
  flex: none; display: grid; place-items: center; width: 30px; height: 30px; border-radius: 8px;
  border: 1.5px solid var(--vien); font-weight: 700; font-size: .9rem; color: var(--chu-mo);
}
.pa .nd { flex: 1; min-width: 0; overflow-wrap: anywhere; }
.pa.chon { border-color: var(--mau); background: var(--mau-nhat); box-shadow: 0 2px 0 var(--mau); }
.pa.chon .nhan { background: var(--mau); border-color: var(--mau); color: #fff; }
.pa:disabled { cursor: default; opacity: 1; }
.pa.dung { border-color: var(--dung); background: var(--dung-nen); box-shadow: 0 2px 0 var(--dung); }
.pa.dung .nhan { background: var(--dung); border-color: var(--dung); color: #fff; }
.pa.sai { border-color: var(--sai); background: var(--sai-nen); box-shadow: 0 2px 0 var(--sai); }
.pa.sai .nhan { background: var(--sai); border-color: var(--sai); color: #fff; }
.pa .dau { font-weight: 700; font-size: 1.1rem; }
.pa.dung .dau { color: var(--dung); }
.pa.sai .dau { color: var(--sai); }

.ds-y { margin-top: 10px; display: grid; gap: 8px; }
.y-dong { display: flex; align-items: center; gap: 10px; padding: 8px 10px; border: 1px solid var(--vien); border-radius: 12px; }
.y-nd { flex: 1; min-width: 0; overflow-wrap: anywhere; }
.y-nhan { font-weight: 700; color: var(--mau-dam); }
.y-nut { display: flex; gap: 6px; flex: none; }
.nut-ds {
  width: 46px; height: 46px; border-radius: 12px; border: 2px solid var(--vien); background: var(--mat);
  font-weight: 700; cursor: pointer; box-shadow: 0 2px 0 var(--vien);
}
.nut-ds.chon { background: var(--mau); border-color: var(--mau); color: #fff; box-shadow: 0 2px 0 var(--mau-dam); }
.nut-ds:disabled { cursor: default; }
.nut-ds:disabled:not(.chon):not(.dung) { opacity: .5; }
.nut-ds.dung { border-color: var(--dung); box-shadow: 0 2px 0 var(--dung); }
.nut-ds.chon.dung { background: var(--dung); color: #fff; }
.nut-ds.sai { background: var(--sai); border-color: var(--sai); color: #fff; box-shadow: 0 2px 0 var(--sai); }
.y-kq { font-size: .85rem; margin-top: 4px; }
.y-kq.ok { color: var(--dung); }
.y-kq.sai { color: var(--sai); }
.y-kq.trong { color: var(--chu-mo); }

.tln { margin-top: 12px; }
.tln-nhan { display: block; font-size: .88rem; color: var(--chu-mo); margin-bottom: 6px; }
.tln-hang { display: flex; gap: 6px; align-items: center; flex-wrap: wrap; }
.tln-o {
  width: 8.5ch; min-height: 52px; padding: 6px 10px; border: 2px solid var(--vien); border-radius: 12px; background: var(--mat);
  font-size: 1.5rem; font-weight: 700; letter-spacing: .18em; text-align: center; font-variant-numeric: tabular-nums;
}
.tln-o:focus { border-color: var(--mau); outline: none; box-shadow: 0 0 0 3px var(--mau-nhat); }
.tln-o::placeholder { color: var(--vien); letter-spacing: .1em; }
.nut-phu { width: 46px; height: 46px; border-radius: 12px; border: 1.5px solid var(--vien); background: var(--mat-2); font-size: 1.2rem; font-weight: 700; cursor: pointer; }
.tln-xem { margin-top: 10px; display: grid; gap: 4px; }
.ok { color: var(--dung); }
.sai { color: var(--sai); }
b.trong { color: var(--chu-mo); }

.kq-nhan { font-size: .8rem; font-weight: 700; padding: 2px 8px; border-radius: 999px; }
.kq-nhan.dung { background: var(--dung-nen); color: var(--dung); }
.kq-nhan.sai { background: var(--sai-nen); color: var(--sai); }
.kq-nhan.mot_phan { background: var(--vang-nen); color: var(--vang); }
.kq-nhan.bo_trong { background: var(--mat-2); color: var(--chu-mo); }
.cau.kq-dung { border-left: 4px solid var(--dung); }
.cau.kq-sai { border-left: 4px solid var(--sai); }
.cau.kq-mot_phan { border-left: 4px solid var(--vang); }

.loi-giai { margin-top: 14px; padding: 12px 14px; border-radius: 12px; background: var(--mat-2); font-size: 1rem; }
.lg-tieu-de { font-weight: 700; margin-bottom: 4px; color: var(--mau-dam); }
.xem-lg { margin-top: 10px; }
.xem-lg summary { cursor: pointer; font-weight: 600; color: var(--mau-dam); min-height: 44px; display: flex; align-items: center; }

/* Đoạn văn chung (Tiếng Anh) */
.nhom-doc { margin-bottom: 16px; }
.doan-dan { background: var(--mat); border: 1px solid var(--vien); border-radius: var(--bo); margin-bottom: 10px; overflow: hidden; }
.doan-dan summary {
  list-style: none; cursor: pointer; display: flex; align-items: center; justify-content: space-between;
  min-height: 48px; padding: 10px 14px; font-weight: 600; background: var(--mau-nhat); color: var(--mau-dam);
}
.doan-dan summary::-webkit-details-marker { display: none; }
.dd-mo::after { content: "Thu gọn ▲"; font-size: .8rem; font-weight: 500; }
.doan-dan:not([open]) .dd-mo::after { content: "Mở lại ▼"; }
.huong-dan { margin: 12px 14px 0; font-style: italic; color: var(--chu-mo); font-size: .92rem; }
.dd-nd { padding: 10px 14px 14px; font-size: 1.0625rem; line-height: 1.7; }

/* ---------- Kết quả ---------- */
.diem-tong { text-align: center; }
.vong-diem {
  --pt: 0;
  width: 150px; height: 150px; margin: 6px auto 4px; border-radius: 50%; display: grid; place-content: center;
  background: radial-gradient(closest-side, var(--mat) 78%, transparent 79%), conic-gradient(var(--mau) calc(var(--pt) * 1%), var(--mat-2) 0);
  grid-auto-flow: column; align-items: baseline;
}
.diem-so { font-size: 2.6rem; font-weight: 700; color: var(--mau-dam); }
.diem-tren { font-size: 1.1rem; color: var(--chu-mo); }
.loi-khen { font-weight: 600; font-size: 1.1rem; margin: 6px 0; }
.diem-tong .hang-nut { justify-content: center; }
.diem-tong.dat .vong-diem { background: radial-gradient(closest-side, var(--mat) 78%, transparent 79%), conic-gradient(var(--dung) calc(var(--pt) * 1%), var(--mat-2) 0); }
.bang-diem { width: 100%; margin: 0; }
.bang-diem td { text-align: left; border: none; border-bottom: 1px solid var(--vien); padding: 10px 4px; }
.bang-diem td.so { text-align: right; white-space: nowrap; }
.cd-kq { padding: 10px 0; border-bottom: 1px solid var(--vien); }
.cd-kq:last-child { border-bottom: none; }
.cdk-dau { display: flex; justify-content: space-between; gap: 10px; align-items: baseline; }
.cdk-ten { font-weight: 500; }
.cdk-so { font-size: .82rem; color: var(--chu-mo); white-space: nowrap; }
.thanh-pt { display: block; flex: 1; height: 8px; border-radius: 4px; background: var(--mat-2); overflow: hidden; margin: 6px 0; }
.thanh-pt span { display: block; height: 100%; background: var(--mau); border-radius: 4px; }
.cd-kq.yeu .thanh-pt span { background: var(--sai); }
.cd-kq.yeu .cdk-ten::before { content: "⚠ "; color: var(--sai); }
.loc { display: flex; gap: 6px; margin-bottom: 10px; }
.loc button { min-height: 40px; padding: 6px 14px; border-radius: 999px; border: 1.5px solid var(--vien); background: var(--mat); font-weight: 600; cursor: pointer; }
.loc button.chon { background: var(--chu); color: var(--nen); border-color: var(--chu); }

/* ---------- Học chuyên đề ---------- */
.khung-buoc { max-width: 720px; margin: 0 auto; padding: 10px 16px 0; }
.buoc { display: flex; gap: 6px; overflow-x: auto; padding-bottom: 4px; scrollbar-width: none; align-items: center; }
.buoc::-webkit-scrollbar { display: none; }
.buoc a {
  flex: none; display: flex; align-items: center; gap: 6px; min-height: 44px; padding: 6px 12px 6px 6px;
  border-radius: 999px; background: var(--mat); border: 1.5px solid var(--vien); text-decoration: none; color: var(--chu-mo);
  font-weight: 600; font-size: .9rem;
}
.b-so { display: grid; place-items: center; width: 28px; height: 28px; border-radius: 50%; background: var(--mat-2); font-size: .82rem; }
.buoc a.xong .b-so { background: var(--dung); color: #fff; }
.buoc a.hien { border-color: var(--mau); color: var(--mau-dam); background: var(--mau-nhat); }
.buoc a.hien .b-so { background: var(--mau); color: #fff; }
.the-kt {
  min-height: 300px; padding: 20px 18px; border-radius: 20px; background: var(--mat); cursor: pointer;
  border: 2px solid var(--vien); box-shadow: 0 4px 0 var(--vien); display: flex; flex-direction: column; margin-top: 12px;
}
.the-kt:not(.lat) { justify-content: center; text-align: center; background: linear-gradient(160deg, var(--mau-nhat), var(--mat)); border-color: color-mix(in srgb, var(--mau) 40%, var(--vien)); }
.the-kt.lat { animation: lat .22s ease-out; }
@keyframes lat { from { transform: rotateY(70deg); opacity: .3; } }
.tk-dem { font-size: .8rem; color: var(--chu-mo); font-weight: 600; }
.the-kt.lat .tk-dem { margin-bottom: 4px; }
.tk-tieu-de { font-size: 1.3rem; margin: 10px 0; }
.the-kt.lat .tk-tieu-de { font-size: 1.15rem; margin: 0 0 8px; color: var(--mau-dam); }
.tk-goi-y { color: var(--chu-mo); font-size: .9rem; margin-top: 14px; }
.tk-goi-y::before { content: "👆 "; }
.tk-nd { font-size: 1.0625rem; }
.loi-hay-gap { margin-top: 14px; padding: 10px 12px; border-radius: 12px; background: var(--vang-nen); border-left: 4px solid var(--vang); }
.loi-hay-gap b { color: var(--vang); }
.the-dieu-huong { justify-content: space-between; flex-wrap: nowrap; }
.cham { display: flex; gap: 5px; flex-wrap: wrap; justify-content: center; min-width: 0; }
.cham i { width: 7px; height: 7px; border-radius: 50%; background: var(--vien); }
.cham i.hien { background: var(--mau); width: 18px; border-radius: 4px; }
.tom-tat { font-size: 1.0625rem; }
.vd { margin-bottom: 18px; }
.vd .cau { margin-bottom: 0; }
.vd > .hang-nut, .vd > .loi-giai { margin: 8px 0 0; }
.tien-do-luyen { display: flex; align-items: center; gap: 10px; margin: 8px 0 12px; font-weight: 600; font-size: .9rem; }
.tien-do-luyen .thanh-pt { margin: 0; }
.phan-hoi { padding: 12px 14px; border-radius: 12px; font-weight: 600; margin-top: -4px; display: flex; align-items: center; gap: 8px; animation: hien .2s; }
.phan-hoi.ok { background: var(--dung-nen); color: var(--dung); }
.phan-hoi.sai { background: var(--sai-nen); color: var(--sai); }
.phan-hoi.mot-phan { background: var(--vang-nen); color: var(--vang); }
.ph-icon { display: grid; place-items: center; width: 28px; height: 28px; border-radius: 50%; background: currentColor; }
.ph-icon { color: inherit; }
.phan-hoi .ph-icon { background: none; border: 2px solid currentColor; font-size: .95rem; }
.tong-ket { text-align: center; }
.tk-lon { font-size: 3rem; font-weight: 700; color: var(--mau-dam); }
.tong-ket .hang-nut { justify-content: center; }
.gioi-thieu h2 { margin-top: 8px; }
.cau-truc { padding-left: 1.2em; }

/* ---------- Sổ lỗi ---------- */
.muc-loi { background: var(--mat); border: 1px solid var(--vien); border-radius: var(--bo); padding: 12px 14px; margin-bottom: 10px; }
.ml-dau { display: flex; flex-wrap: wrap; gap: 6px; }
.ml-tom-tat { margin: 8px 0; }
.ml-vung .cau { border: none; padding: 0; margin-top: 8px; }

/* ---------- Đang tải ---------- */
.dang-tai { display: flex; align-items: center; justify-content: center; gap: 10px; min-height: 50vh; color: var(--chu-mo); }
.xoay { width: 22px; height: 22px; border-radius: 50%; border: 3px solid var(--vien); border-top-color: var(--mau); animation: xoay .8s linear infinite; }
@keyframes xoay { to { transform: rotate(360deg); } }

/* ---------- Màn hình rất hẹp ---------- */
@media (max-width: 400px) {
  .khung { padding-left: 12px; padding-right: 12px; }
  .cau { padding: 12px 10px 14px; }
  .pa-ds.luoi { grid-template-columns: 1fr; }
  .y-dong { padding: 8px; gap: 8px; }
  .nut-ds { width: 44px; height: 44px; }
  .thanh-de .nut.nho { padding: 6px 10px; }
}
@media (prefers-reduced-motion: reduce) {
  *, *::before, *::after { animation: none !important; transition: none !important; scroll-behavior: auto !important; }
}

/* ---------- H-SCA: phát biểu đánh số ---------- */
.tm-nhom { align-self: flex-start; font-size: .72rem; font-weight: 700; color: color-mix(in srgb, var(--mau-mon) 80%, var(--chu)); background: color-mix(in srgb, var(--mau-mon) 16%, var(--mat)); padding: 2px 8px; border-radius: 999px; }
.nhom-thi { margin: -4px 4px 8px; }
.phan-hd { flex-basis: 100%; font-size: .85rem; color: var(--chu-mo); line-height: 1.45; }
.pb-huong-dan { margin: 12px 0 0; padding: 8px 12px; border-radius: 10px; background: var(--mau-nhat); color: var(--chu); font-size: .95rem; line-height: 1.45; }
.pb-huong-dan::before { content: "👆 "; }
.pb-ds { display: grid; gap: 10px; margin: 10px 0 8px; }
.pb {
  display: flex; align-items: flex-start; gap: 10px; width: 100%; min-height: 56px; padding: 12px 12px;
  text-align: left; border: 2px solid var(--vien); border-radius: 14px; background: var(--mat); color: var(--chu);
  cursor: pointer; box-shadow: 0 2px 0 var(--vien); transition: border-color .12s, background-color .12s;
}
@media (hover: hover) { .pb:not(.chon):not([aria-disabled]):hover { border-color: color-mix(in srgb, var(--mau) 50%, var(--vien)); } }
.pb[aria-disabled] { cursor: default; }
.pb .nd { flex: 1; min-width: 0; overflow-wrap: anywhere; }
.pb-so { flex: none; display: grid; place-items: center; min-width: 26px; height: 26px; padding: 0 6px; border-radius: 8px; background: var(--mat-2); font-weight: 700; font-size: .85rem; margin-top: 1px; }
/* Ô trạng thái: rỗng (○) khi chưa chọn, đặc có ✓ khi chọn, ✗ khi chọn sai (xem lại) */
.o-tick {
  flex: none; display: grid; place-items: center; width: 28px; height: 28px; border-radius: 50%;
  border: 2.5px solid var(--chu-mo); background: transparent; font-weight: 800; font-size: 1rem; color: #fff;
}
.pb.chon { border-color: var(--mau); background: var(--mau-nhat); box-shadow: 0 2px 0 var(--mau-dam); }
.pb.chon .o-tick { background: var(--mau); border-color: var(--mau); }
.pb.chon .pb-so { background: var(--mau); color: #fff; }
.pb-tom-tat { display: flex; flex-wrap: wrap; align-items: center; justify-content: space-between; gap: 8px; margin: 4px 0 6px; font-size: .95rem; color: var(--chu-mo); }
.pb-tom-tat b { color: var(--chu); }
/* Xem lại */
.pb.pb-dung-chon { border-color: var(--dung); background: var(--dung-nen); box-shadow: 0 2px 0 var(--dung); }
.pb.pb-dung-chon .o-tick { background: var(--dung); border-color: var(--dung); color: var(--mat); }
.pb.pb-bo-sot { border-style: dashed; border-color: var(--dung); box-shadow: none; }
.pb.pb-bo-sot .o-tick { border-color: var(--dung); border-style: dashed; }
.pb.pb-chon-sai { border-color: var(--sai); background: var(--sai-nen); box-shadow: 0 2px 0 var(--sai); }
.pb.pb-chon-sai .o-tick { background: var(--sai); border-color: var(--sai); color: var(--mat); }
.pb.pb-trung-tinh { opacity: .8; box-shadow: none; }
.pb-nhan { flex: none; align-self: center; font-size: .78rem; font-weight: 700; padding: 3px 8px; border-radius: 999px; white-space: nowrap; }
.pb-nhan.ok { background: var(--dung); color: var(--mat); }
.pb-nhan.bo-sot { border: 1.5px dashed var(--dung); color: var(--dung); }
.pb-nhan.sai { background: var(--sai); color: var(--mat); }
.tln-o.dai { width: 11ch; letter-spacing: .06em; }
.vb sup, .vb sub, .nd sup, .nd sub, .khung-tieu-de sup, td sup, td sub { line-height: 0; }

/* ---------- Tự quảng cáo ---------- */
.qc {
  --mau-qc: var(--mau);
  position: relative; display: flex; flex-wrap: wrap; align-items: center; gap: 10px 14px;
  margin: 14px 0; padding: 12px 44px 12px 14px; border-radius: var(--bo);
  background: color-mix(in srgb, var(--mau-qc) 9%, var(--mat));
  border: 1.5px solid color-mix(in srgb, var(--mau-qc) 35%, var(--vien));
}
.qc-nd { flex: 1 1 220px; min-width: 0; display: flex; flex-direction: column; gap: 2px; }
.qc-td { color: color-mix(in srgb, var(--mau-qc) 80%, var(--chu)); }
.qc-mo { font-size: .9rem; color: var(--chu-mo); line-height: 1.45; }
.qc-nut { background: var(--mau-qc); border-color: var(--mau-qc); color: #fff; }
.qc-an {
  position: absolute; top: 2px; right: 2px; width: 40px; height: 40px; border: none; background: transparent;
  color: var(--chu-mo); font-size: 1rem; border-radius: 10px; cursor: pointer;
}
.qc-an:hover { background: var(--mat-2); }

/* Chống tràn ngang: chuỗi TeX thô dài (khi MathJax chưa tải) xuống dòng; công thức quá rộng cuộn trong khung câu. */
.vb, .nd, .y-nd, .tk-tieu-de { overflow-wrap: anywhere; }
.cau, .the-kt, .tom-tat, .loi-giai, .muc-loi { overflow-x: auto; }

/* ---------- Công thức dựng sẵn (SVG từ tools/tex2svg.mjs) ---------- */
.cth > svg { display: inline-block; overflow: visible; max-width: none; fill: currentColor; stroke: currentColor; stroke-width: 0; }
.cth-k { display: block; text-align: center; overflow-x: auto; overflow-y: hidden; margin: .4em 0; padding: 2px 0; }
/* Ô bảng: không bẻ chữ từng kí tự (bảng đã nằm trong khung cuộn ngang) */
.cuon table th, .cuon table td { overflow-wrap: normal; word-break: normal; min-width: 3.5em; }
.cat-dong { display: -webkit-box; -webkit-line-clamp: 3; -webkit-box-orient: vertical; overflow: hidden; }

/* Vừa khít bề ngang điện thoại: không có thanh trượt ngang trong nội dung */
.cth-k { overflow: visible; }
.cau, .the-kt, .tom-tat, .loi-giai, .muc-loi, .dd-nd { overflow-x: hidden; }
.cuon { overflow-x: auto; }               /* chỉ bảng biểu mới được trượt */
.cth { white-space: nowrap; }
.nd, .vb, .dd-nd, .loi-giai p, .loi-giai li { line-height: 1.75; }

/* ---------- Phần thưởng & tương tác ---------- */
.the-tien-do { display: grid; grid-template-columns: auto 1fr auto; gap: 8px 14px; align-items: center; margin: 12px 0; padding: 14px;
  background: var(--mat); border: 1px solid var(--vien); border-radius: var(--bo); box-shadow: var(--bong); color: var(--chu); text-decoration: none; }
.vong { --tl: 0; width: 68px; height: 68px; border-radius: 50%; display: grid; place-content: center; text-align: center; line-height: 1.05;
  background: radial-gradient(closest-side, var(--mat) 78%, transparent 80% 100%), conic-gradient(var(--dung) calc(var(--tl) * 1%), var(--mat-2) 0); }
.vong b { font-size: 1.15rem; }
.vong small { font-size: .62rem; color: var(--chu-mo); }
.ttd-giua { display: flex; flex-direction: column; gap: 4px; min-width: 0; }
.ttd-cap { font-size: .95rem; }
.thanh-xp { display: block; height: 10px; border-radius: 5px; background: var(--mat-2); overflow: hidden; }
.thanh-xp > span { display: block; height: 100%; border-radius: 5px; background: linear-gradient(90deg, #7DB7FF, var(--mau)); transition: width .6s cubic-bezier(.2, .8, .2, 1); }
.ttd-phu { font-size: .78rem; color: var(--chu-mo); }
.ttd-gem { display: flex; align-items: center; gap: 4px; font-size: 1.1rem; padding: 6px 10px; border-radius: 999px; background: var(--mau-nhat); }
.ttd-gem b { font-size: 1rem; }
.tuan { grid-column: 1 / -1; display: grid; grid-template-columns: repeat(7, 1fr); gap: 4px; text-align: center; font-size: .72rem; color: var(--chu-mo); }
.tuan-o { display: flex; flex-direction: column; align-items: center; gap: 2px; }
.tuan-o i { width: 30px; height: 30px; border-radius: 50%; display: grid; place-items: center; font-style: normal; font-size: 1rem; background: var(--mat-2); }
.tuan-o.hoc i { background: color-mix(in srgb, #FFB020 22%, var(--mat)); }
.tuan-o.bv i { background: var(--mau-nhat); }
.tuan-o.nay { font-weight: 700; color: var(--chu); }
.tuan-o.nay i { outline: 2px solid var(--mau); outline-offset: 1px; }

.xp-chip { display: inline-block; margin-left: auto; padding: 2px 10px; border-radius: 999px; background: var(--dung); color: #fff; font-size: .82rem; font-weight: 700; animation: bay-len .7s ease-out; }
.xp-chip.combo { margin-left: 6px; background: #FF8A1F; }
.xp-chip.gem { background: var(--mau); }
.phan-hoi.ok { animation: nay .35s ease-out; }
.phan-hoi.sai { animation: rung-sai .35s; }
.thuong-dong { display: flex; justify-content: center; gap: 8px; }
.thuong-dong .xp-chip { margin: 0; font-size: 1rem; padding: 4px 14px; }
.sao-hang { display: flex; justify-content: center; gap: 6px; font-size: 2.4rem; line-height: 1; margin: 6px 0; }
.sao { color: var(--mat-2); text-shadow: 0 0 0 var(--vien); }
.sao.sang { color: #F2B544; animation: sao-nay .5s var(--d, 0s) both cubic-bezier(.3, 1.6, .5, 1); }
.linh-vat { display: block; margin: 0 auto; animation: nhun 2.4s ease-in-out infinite; }
.linh-vat.buon { animation: none; }

.phao-giay { position: fixed; inset: 0; width: 100%; height: 100%; z-index: 120; pointer-events: none; }
.hop-mung { text-align: center; padding-top: 12px; }
.hop-mung .mung-icon { font-size: 2.6rem; animation: sao-nay .6s both cubic-bezier(.3, 1.6, .5, 1); }
.hop-mung .nut { width: 100%; margin-top: 8px; }

.tt-dau { display: flex; align-items: center; gap: 14px; }
.tt-dau h2 { margin: 0; font-size: 1.2rem; }
.tt-dau .mo { margin: 2px 0 0; }
.ds-hh { list-style: none; margin: 0; padding: 0; display: grid; grid-template-columns: repeat(auto-fill, minmax(140px, 1fr)); gap: 10px; }
.hh { display: flex; flex-direction: column; align-items: center; gap: 2px; text-align: center; padding: 12px 8px; border-radius: var(--bo);
  background: var(--mat); border: 1px solid var(--vien); opacity: .6; }
.hh.co { opacity: 1; border-color: #F2B544; background: var(--vang-nen); }
.hh-icon { font-size: 2rem; line-height: 1.2; }
.hh-ten { font-weight: 700; font-size: .9rem; }
.hh-mt { font-size: .76rem; color: var(--chu-mo); }

@keyframes bay-len { from { opacity: 0; transform: translateY(8px) scale(.8); } to { opacity: 1; transform: none; } }
@keyframes nay { 0% { transform: scale(.96); } 50% { transform: scale(1.03); } 100% { transform: scale(1); } }
@keyframes rung-sai { 0%, 100% { transform: translateX(0); } 25% { transform: translateX(-5px); } 75% { transform: translateX(5px); } }
@keyframes sao-nay { from { transform: scale(0) rotate(-40deg); opacity: 0; } to { transform: none; opacity: 1; } }
@keyframes nhun { 0%, 100% { transform: translateY(0); } 50% { transform: translateY(-4px); } }
@media (prefers-reduced-motion: reduce) {
  .xp-chip, .phan-hoi, .sao.sang, .linh-vat, .hop-mung .mung-icon { animation: none; }
  .thanh-xp > span { transition: none; }
}

/* Bảng xếp chồng trên điện thoại */
@media (max-width: 640px) {
  .bang-chong, .bang-chong tbody, .bang-chong tr, .bang-chong td, .bang-chong th { display: block; width: 100%; max-width: 100%; min-width: 0 !important; }
  .bang-chong thead { position: absolute; width: 1px; height: 1px; overflow: hidden; clip-path: inset(50%); }
  .bang-chong tr { border: 1px solid var(--vien); border-radius: 12px; margin: 0 0 10px; padding: 4px 12px; background: var(--mat); }
  .bang-chong td { border: 0 !important; border-bottom: 1px dashed var(--vien) !important; padding: 8px 0 !important; overflow-wrap: anywhere; }
  .bang-chong td:last-child { border-bottom: 0 !important; }
  .bang-chong td[data-nhan]::before { content: attr(data-nhan); display: block; font-size: .72rem; font-weight: 700; letter-spacing: .03em;
    text-transform: uppercase; color: var(--chu-mo); margin-bottom: 2px; }
  .bang-chong td:first-child { font-weight: 600; }
}

/* Ô Đúng/Sai: màn hình hẹp thì nút Đ/S xuống dưới nội dung (để công thức dài có đủ chỗ) */
@media (max-width: 480px) {
  .y-dong { flex-wrap: wrap; }
  .y-dong .y-nd { flex: 1 1 100%; }
  .y-dong .y-nut { margin-left: auto; }
}

/* Phương án trả lời: không bao giờ rộng hơn khung chứa */
.pa { max-width: 100%; min-width: 0; }
.pa .nd { flex: 1 1 0; }
.pa-ds.luoi > * { min-width: 0; }

/* Nút có chữ dài (ví dụ "Bảo vệ chuỗi hôm qua") được xuống dòng, không tràn màn hình */
.nut { max-width: 100%; white-space: normal; }

/* ---------- Tài khoản, gói Free/Pro, khoá ---------- */
.the-goi { font-weight: 700; }
.the-goi.pro { background: linear-gradient(90deg, #F2B544, #FF8A1F); color: #2B1B00; border-color: transparent; }
.the-goi.lon { display: inline-block; padding: 1px 10px; border-radius: 999px; background: var(--mat-2); }
.the-goi.lon.pro { color: #2B1B00; }
.hop.tk, .hop-nen.tk .hop { max-height: calc(100% - 24px); overflow-y: auto; text-align: center; }
.hop-nen.tk .hop > svg { margin-bottom: 4px; }
.tk-tab { display: grid; grid-template-columns: 1fr 1fr; gap: 4px; padding: 4px; margin: 10px 0; border-radius: 12px; background: var(--mat-2); }
.tk-tab button { border: 0; background: none; padding: 8px; border-radius: 9px; font: inherit; font-weight: 600; color: var(--chu-mo); cursor: pointer; }
.tk-tab button[aria-selected="true"] { background: var(--mat); color: var(--chu); box-shadow: var(--bong); }
.tk-form { display: grid; gap: 10px; text-align: left; margin: 8px 0; }
.tk-form label { display: grid; gap: 4px; font-size: .85rem; font-weight: 600; color: var(--chu-mo); }
.tk-form input { font: inherit; font-size: 1rem; padding: 11px 12px; border: 2px solid var(--vien); border-radius: 12px; background: var(--mat); color: var(--chu); min-width: 0; width: 100%; }
.tk-form input:focus { outline: none; border-color: var(--mau); }
.tk-tt { min-height: 1.4em; margin: 4px 0; }
.tk-tt.loi { color: var(--sai); font-weight: 600; }
.mo.nho { font-size: .78rem; }
.nut.lien-ket { border: 0; background: none; box-shadow: none; color: var(--mau-dam); text-decoration: underline; min-height: 0; padding: 4px; }
.o-de.khoa { opacity: .72; }
.o-de.khoa .od-diem::before { content: "🔒 "; }
.nhan-pro.mo-khoa { background: color-mix(in srgb, #F2B544 25%, var(--mat)); color: var(--vang); }
.khoa-pro { text-align: center; }
.khoa-pro h2 { font-size: 1.15rem; }
.khoa-pro .hang-nut { justify-content: center; }
.so-sanh { width: 100%; border-collapse: collapse; font-size: .92rem; }
.so-sanh th, .so-sanh td { padding: 8px 6px; border-bottom: 1px solid var(--vien); text-align: center; }
.so-sanh th:first-child, .so-sanh td:first-child { text-align: left; }