:root {
  --black: #111;
  --gray-900: #1a1a1a;
  --gray-700: #3a3a3a;
  --gray-600: #555;
  --gray-500: #777;
  --gray-400: #999;
  --gray-300: #bbb;
  --gray-200: #e0e0e0;
  --gray-150: #eaeaea;
  --gray-100: #f5f5f5;
  --gray-50: #fafafa;
  --white: #fff;
  --green: #16a34a;
  --blue: #2563eb;
  --amber: #d97706;
  --red: #dc2626;
  --radius-sm: 6px;
  --radius-md: 8px;
  --radius-lg: 14px;
  --shadow-sm: 0 1px 2px rgba(0, 0, 0, .04), 0 1px 8px rgba(0, 0, 0, .03);
  --shadow-md: 0 2px 8px rgba(0, 0, 0, .06);
  --transition: all .16s ease;
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  background: var(--white);
  color: var(--black);
  font-family: Inter, -apple-system, BlinkMacSystemFont, "Segoe UI", "Microsoft YaHei", sans-serif;
  line-height: 1.6;
}

a {
  color: inherit;
  text-decoration: none;
}

button,
input {
  font: inherit;
}

.site-header {
  position: sticky;
  top: 0;
  z-index: 10;
  border-bottom: 1px solid var(--gray-150);
  background: rgba(255, 255, 255, .92);
  backdrop-filter: blur(12px);
}

.header-inner {
  width: min(1180px, calc(100% - 40px));
  height: 64px;
  margin: 0 auto;
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 24px;
}

.brand {
  display: flex;
  align-items: center;
  gap: 10px;
  font-weight: 700;
  letter-spacing: 0;
}

.brand-mark {
  width: 32px;
  height: 32px;
  border: 1px solid var(--gray-200);
  border-radius: var(--radius-sm);
  display: grid;
  place-items: center;
  background: var(--black);
  color: var(--white);
  font-size: 13px;
}

.main-nav {
  display: flex;
  align-items: center;
  gap: 18px;
  color: var(--gray-600);
  font-size: 14px;
}

.main-nav a:hover {
  color: var(--black);
}

.container {
  width: min(1180px, calc(100% - 40px));
  margin: 0 auto;
}

.hero {
  padding: 46px 0 26px;
  border-bottom: 1px solid var(--gray-150);
}

.eyebrow {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin-bottom: 14px;
  color: var(--gray-600);
  font-size: 13px;
  font-weight: 600;
}

.eyebrow::before {
  content: "";
  width: 16px;
  height: 1px;
  border-radius: 999px;
  background: var(--gray-400);
}

h1 {
  margin: 0;
  max-width: 880px;
  font-size: 40px;
  line-height: 1.18;
  letter-spacing: 0;
}

.hero-desc {
  max-width: 760px;
  margin: 14px 0 0;
  color: var(--gray-600);
  font-size: 16px;
}

.hero-meta {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  margin-top: 22px;
}

.content-byline {
  display: flex;
  flex-wrap: wrap;
  gap: 8px 16px;
  margin-top: 16px;
  color: var(--gray-500);
  font-size: 13px;
}

.content-byline a {
  color: var(--gray-700);
  font-weight: 650;
}

.content-byline a:hover {
  color: var(--black);
}

.hero-actions {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
  margin-top: 24px;
}

.btn-primary,
.btn-outline {
  display: inline-flex;
  align-items: center;
  justify-content: center;
  min-height: 40px;
  border-radius: var(--radius-sm);
  padding: 9px 16px;
  font-size: 14px;
  font-weight: 650;
  transition: var(--transition);
}

.btn-primary {
  background: var(--black);
  color: var(--white);
}

.btn-primary:hover {
  background: var(--gray-800);
}

.btn-outline {
  border: 1px solid var(--gray-200);
  color: var(--gray-700);
  background: var(--white);
}

.btn-outline:hover {
  border-color: var(--gray-300);
  color: var(--black);
  box-shadow: var(--shadow-sm);
}

.meta-pill,
.tag,
.level,
.difficulty {
  display: inline-flex;
  align-items: center;
  border-radius: 999px;
  background: var(--gray-100);
  color: var(--gray-700);
  font-size: 12px;
  font-weight: 600;
  line-height: 1;
}

.meta-pill {
  padding: 8px 12px;
}

.tag,
.level,
.difficulty {
  padding: 5px 8px;
}

.difficulty.medium,
.difficulty.中等 {
  background: #fef3c7;
  color: #92400e;
}

.difficulty.advanced,
.difficulty.进阶 {
  background: #fee2e2;
  color: #991b1b;
}

.difficulty.medium {
  background: var(--gray-100);
  color: var(--gray-700);
}

.difficulty.advanced {
  background: var(--gray-900);
  color: var(--white);
}

.layout {
  display: grid;
  grid-template-columns: minmax(0, 1fr) 300px;
  gap: 28px;
  padding: 30px 0 58px;
}

.main-stack,
.side-stack {
  display: flex;
  flex-direction: column;
  gap: 18px;
}

.side-stack {
  position: sticky;
  top: 84px;
  align-self: start;
  max-height: calc(100vh - 104px);
  overflow-y: auto;
  padding-right: 2px;
}

.panel {
  border: 1px solid var(--gray-150);
  border-radius: var(--radius-md);
  background: var(--white);
  overflow: hidden;
}

.panel-head {
  min-height: 58px;
  padding: 16px 18px;
  border-bottom: 1px solid var(--gray-150);
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 14px;
}

.panel-title {
  margin: 0;
  font-size: 16px;
  line-height: 1.35;
}

.panel-sub {
  color: var(--gray-500);
  font-size: 12px;
}

.search {
  width: min(320px, 100%);
  border: 1px solid var(--gray-200);
  border-radius: var(--radius-sm);
  padding: 8px 11px;
  outline: none;
}

.search:focus {
  border-color: var(--gray-400);
  box-shadow: 0 0 0 3px rgba(0, 0, 0, .04);
}

.subject-groups {
  display: grid;
  gap: 18px;
  padding: 14px;
}

.subject-group {
  display: grid;
  gap: 12px;
}

.subject-group-head {
  display: flex;
  align-items: center;
  justify-content: space-between;
  gap: 12px;
  padding: 2px 2px 0;
}

.subject-group-head h3 {
  margin: 0;
  font-size: 15px;
}

.subject-group-head span {
  color: var(--gray-500);
  font-size: 12px;
}

.subject-grid {
  display: grid;
  grid-template-columns: repeat(3, minmax(0, 1fr));
  gap: 12px;
}

.subject-card {
  border: 1px solid var(--gray-150);
  border-radius: var(--radius-md);
  padding: 16px;
  transition: var(--transition);
}

.subject-card.is-empty {
  background: var(--gray-50);
}

.subject-card:hover,
.question-row:hover,
.quick-link:hover {
  border-color: var(--gray-300);
  box-shadow: var(--shadow-sm);
  transform: translateY(-1px);
}

.subject-name {
  margin: 0 0 8px;
  font-size: 16px;
}

.subject-top {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 10px;
  margin-bottom: 8px;
}

.subject-top .subject-name {
  margin: 0;
}

.subject-meta {
  display: flex;
  flex-wrap: wrap;
  gap: 6px;
  color: var(--gray-500);
  font-size: 12px;
}

.subject-summary {
  min-height: 44px;
  margin: 10px 0 0;
  color: var(--gray-600);
  font-size: 13px;
  line-height: 1.65;
}

.question-list {
  display: flex;
  flex-direction: column;
}

.question-row {
  display: grid;
  grid-template-columns: 1fr auto;
  gap: 16px;
  padding: 16px 18px;
  border-bottom: 1px solid var(--gray-150);
  transition: var(--transition);
}

.question-row:last-child {
  border-bottom: 0;
}

.question-title {
  display: block;
  margin-bottom: 7px;
  font-weight: 650;
  line-height: 1.45;
}

.question-stem {
  margin: 0;
  color: var(--gray-600);
  font-size: 14px;
}

.question-meta {
  display: flex;
  flex-wrap: wrap;
  gap: 7px;
  margin-top: 10px;
}

.row-side {
  min-width: 74px;
  text-align: right;
  color: var(--gray-500);
  font-size: 12px;
}

.side-card {
  border: 1px solid var(--gray-150);
  border-radius: var(--radius-md);
  padding: 14px;
  background: var(--gray-50);
  transition: var(--transition);
}

.side-card:hover {
  border-color: var(--gray-200);
}

.side-card h2,
.side-card h3 {
  margin: 0 0 12px;
  color: var(--gray-900);
  font-size: 12px;
  font-weight: 700;
  text-transform: uppercase;
  letter-spacing: .04em;
}

.quick-list {
  display: flex;
  flex-direction: column;
  gap: 7px;
}

.quick-link {
  border: 0;
  border-radius: var(--radius-sm);
  padding: 8px 10px;
  background: var(--white);
  color: var(--gray-700);
  font-size: 13px;
  line-height: 1.45;
  transition: var(--transition);
}

.quick-link:hover {
  background: var(--gray-100);
  color: var(--black);
}

.empty-state {
  padding: 36px 24px;
  text-align: center;
}

.empty-state h3 {
  margin: 0 0 8px;
  font-size: 18px;
}

.empty-state p {
  max-width: 520px;
  margin: 0 auto 18px;
  color: var(--gray-600);
  font-size: 14px;
}

.guide-list {
  display: grid;
  gap: 10px;
}

.guide-list div {
  display: grid;
  grid-template-columns: 28px 1fr;
  gap: 10px;
  align-items: center;
  border: 1px solid var(--gray-150);
  border-radius: var(--radius-sm);
  padding: 10px;
}

.guide-list strong {
  width: 28px;
  height: 28px;
  display: grid;
  place-items: center;
  border-radius: var(--radius-sm);
  background: var(--black);
  color: var(--white);
  font-size: 12px;
}

.guide-list span {
  color: var(--gray-700);
  font-size: 13px;
}

.point-cloud {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
}

.point-chip {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  border: 1px solid var(--gray-150);
  border-radius: 999px;
  padding: 6px 9px;
  color: var(--gray-700);
  font-size: 12px;
  font-weight: 600;
}

.point-chip em {
  color: var(--gray-400);
  font-style: normal;
  font-weight: 700;
}

.crumbs {
  display: flex;
  flex-wrap: wrap;
  gap: 8px;
  margin-bottom: 18px;
  color: var(--gray-500);
  font-size: 13px;
}

.crumbs a:hover {
  color: var(--black);
}

.article {
  padding: 0;
}

article.panel {
  border: 0;
  border-radius: 0;
  background: transparent;
  overflow: visible;
}

.question-card {
  padding: 0 0 28px;
  border-bottom: 1px solid var(--gray-150);
  background: transparent;
}

.question-label {
  display: inline-flex;
  align-items: center;
  gap: 6px;
  margin-bottom: 12px;
  border: 0;
  border-radius: 999px;
  padding: 0;
  background: transparent;
  color: var(--gray-500);
  font-size: 12px;
  font-weight: 700;
}

.question-label::before {
  content: "";
  width: 6px;
  height: 6px;
  border-radius: 50%;
  background: var(--gray-500);
}

.stem {
  margin: 0;
  color: var(--gray-900);
  font-size: 21px;
  line-height: 1.7;
  letter-spacing: 0;
}

.options {
  display: grid;
  gap: 8px;
  margin-top: 22px;
}

.option {
  display: grid;
  grid-template-columns: 34px 1fr;
  gap: 10px;
  align-items: start;
  border: 0;
  border-left: 3px solid transparent;
  border-radius: var(--radius-md);
  padding: 10px 12px 10px 9px;
  background: var(--gray-50);
  transition: var(--transition);
}

.option:hover {
  background: var(--gray-100);
  color: var(--black);
  box-shadow: none;
}

.option.correct {
  border-left-color: var(--black);
  background: var(--white);
  box-shadow: inset 0 0 0 1px var(--gray-150);
}

.option-key {
  width: 30px;
  height: 30px;
  display: grid;
  place-items: center;
  border-radius: var(--radius-sm);
  background: var(--white);
  color: var(--gray-900);
  font-weight: 700;
  line-height: 1;
  box-shadow: inset 0 0 0 1px var(--gray-150);
}

.option.correct .option-key {
  background: var(--black);
  color: var(--white);
  box-shadow: none;
}

.content-note {
  position: relative;
  display: grid;
  gap: 4px;
  margin-top: 20px;
  border: 0;
  border-left: 2px solid var(--gray-200);
  border-radius: 0;
  padding: 9px 0 9px 14px;
  background: transparent;
  color: var(--gray-600);
  font-size: 13px;
  line-height: 1.65;
}

.content-note::before {
  content: none;
}

.content-note strong {
  color: var(--gray-900);
}

.answer-box {
  margin: 30px 0 0;
  border: 0;
  border-left: 4px solid var(--black);
  border-radius: var(--radius-md);
  padding: 18px 20px;
  background: var(--gray-50);
  box-shadow: none;
}

.answer-head {
  display: flex;
  align-items: flex-start;
  justify-content: space-between;
  gap: 14px;
  margin-bottom: 10px;
}

.answer-label {
  color: var(--gray-500);
  font-size: 12px;
  font-weight: 700;
}

.answer-value {
  margin: 4px 0 10px;
  font-size: 30px;
  font-weight: 800;
  line-height: 1;
}

.explanation {
  margin: 0;
  color: var(--gray-700);
  line-height: 1.85;
}

.article-guide {
  position: sticky;
  top: 64px;
  z-index: 2;
  display: flex;
  flex-wrap: wrap;
  align-items: center;
  gap: 8px;
  margin: 28px 0 0;
  border: 0;
  border-bottom: 1px solid var(--gray-150);
  border-radius: 0;
  padding: 0 0 10px;
  background: rgba(255, 255, 255, .94);
  backdrop-filter: blur(10px);
}

.article-guide span {
  padding: 0 8px;
  color: var(--gray-500);
  font-size: 12px;
  font-weight: 700;
}

.article-guide a {
  border-radius: 4px;
  padding: 6px 12px;
  color: var(--gray-700);
  font-size: 12px;
  font-weight: 600;
}

.article-guide a:hover {
  background: var(--gray-100);
  color: var(--black);
}

.article-section {
  margin: 34px 0 0;
}

.article-section + .article-section {
  border-top: 0;
  padding-top: 0;
}

.section-kicker {
  display: inline-flex;
  align-items: center;
  gap: 8px;
  margin-bottom: 12px;
  color: var(--gray-500);
  font-size: 12px;
  font-weight: 800;
  letter-spacing: .06em;
}

.section-kicker::before {
  content: "";
  width: 18px;
  height: 1px;
  border-radius: 999px;
  background: var(--gray-400);
}

.content-block {
  margin-top: 20px;
}

.content-block h2,
.answer-summary h2 {
  position: relative;
  display: block;
  margin: 0 0 10px;
  padding-left: 12px;
  font-size: 16px;
}

.content-block h2::before,
.answer-summary h2::before {
  content: "";
  position: absolute;
  left: 0;
  top: .38em;
  width: 3px;
  height: 15px;
  border: 0;
  border-radius: 2px;
  background: var(--gray-300);
}

.content-block p {
  margin: 0;
  color: var(--gray-700);
  line-height: 1.85;
}

.content-block ul {
  margin: 0;
  padding-left: 20px;
  color: var(--gray-700);
}

.content-block li + li {
  margin-top: 7px;
}

.answer-summary {
  border: 0;
  border-left: 4px solid var(--black);
  border-radius: var(--radius-md);
  padding: 18px;
  background: var(--gray-50);
}

.answer-summary h2 {
  color: var(--black);
}

.answer-summary h2::before,
.article-summary h2::before {
  background: var(--black);
}

.memory-tip {
  border: 0;
  border-left: 3px solid var(--gray-300);
  border-radius: var(--radius-md);
  padding: 16px 18px;
  background: var(--gray-50);
}

.memory-tip h2 {
  color: var(--black);
}

.search-intents ul {
  display: grid;
  gap: 8px;
  padding-left: 0;
  list-style: none;
}

.search-intents li {
  border: 0;
  border-bottom: 1px solid var(--gray-150);
  border-radius: 0;
  padding: 10px 0;
  background: transparent;
}

.article-summary {
  border: 0;
  border-left: 3px solid var(--black);
  border-radius: var(--radius-md);
  padding: 16px 18px;
  background: var(--gray-50);
}

.article-summary h2 {
  color: var(--black);
}

.detail-block {
  margin-top: 12px;
  border: 0;
  border-top: 1px solid var(--gray-150);
  border-radius: 0;
  padding: 0;
  background: transparent;
}

.detail-block summary {
  display: flex;
  align-items: center;
  gap: 8px;
  cursor: pointer;
  padding: 12px 0;
  color: var(--gray-900);
  font-weight: 700;
}

.detail-block summary::before {
  content: "+";
  width: 20px;
  height: 20px;
  flex: 0 0 20px;
  display: grid;
  place-items: center;
  border: 0;
  border-radius: var(--radius-sm);
  background: var(--gray-100);
  color: var(--gray-600);
  font-size: 13px;
  line-height: 1;
}

.detail-block[open] summary::before {
  content: "-";
  color: var(--black);
}

.detail-block p,
.detail-block ul {
  margin: 0;
  border-top: 0;
  padding: 0 0 16px 28px;
  color: var(--gray-700);
  line-height: 1.85;
}

.table-wrap {
  width: 100%;
  overflow-x: auto;
}

.compare-table {
  width: 100%;
  min-width: 620px;
  border-collapse: collapse;
  color: var(--gray-700);
  font-size: 13px;
  line-height: 1.6;
}

.compare-table th,
.compare-table td {
  border: 0;
  border-bottom: 1px solid var(--gray-150);
  padding: 11px 10px;
  text-align: left;
  vertical-align: top;
}

.compare-table th {
  background: var(--gray-50);
  color: var(--gray-900);
  font-weight: 700;
}

.compare-table tbody tr:first-child td {
  background: var(--gray-50);
  color: var(--gray-900);
  font-weight: 500;
}

.option-analysis {
  display: grid;
  gap: 10px;
}

.oa-row {
  display: grid;
  grid-template-columns: 34px 1fr;
  gap: 10px;
  border: 0;
  border-top: 1px solid var(--gray-150);
  border-radius: 0;
  padding: 11px 0;
  background: transparent;
}

.oa-row.correct {
  border-color: var(--gray-150);
  background: transparent;
}

.oa-row strong {
  width: 28px;
  height: 28px;
  display: grid;
  place-items: center;
  border-radius: var(--radius-sm);
  background: var(--gray-100);
}

.oa-row.correct strong {
  background: var(--black);
  color: var(--white);
}

.oa-row p {
  margin: 0;
  color: var(--gray-700);
  font-size: 14px;
}

.pager {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 10px;
  padding: 26px 0 0;
  border-top: 1px solid var(--gray-150);
}

.pager a {
  border: 0;
  border-radius: 0;
  padding: 0;
  color: var(--gray-700);
  font-size: 13px;
}

.pager a:last-child {
  text-align: right;
}

.footer {
  border-top: 1px solid var(--gray-150);
  padding: 24px 0;
  color: var(--gray-500);
  font-size: 13px;
}

.footer .container {
  display: flex;
  flex-wrap: wrap;
  gap: 8px 14px;
  align-items: center;
}

.footer strong {
  color: var(--black);
}

.footer a {
  color: var(--gray-600);
}

.footer a:hover {
  color: var(--black);
}

@media (max-width: 920px) {
  .layout {
    grid-template-columns: 1fr;
  }

  .side-stack {
    position: static;
    max-height: none;
    overflow: visible;
    padding-right: 0;
  }

  .subject-grid {
    grid-template-columns: repeat(2, minmax(0, 1fr));
  }

  h1 {
    font-size: 32px;
  }
}

@media (max-width: 640px) {
  .header-inner,
  .container {
    width: min(100% - 28px, 1180px);
  }

  .header-inner {
    height: auto;
    padding: 14px 0;
    align-items: flex-start;
    flex-direction: column;
    gap: 10px;
  }

  .main-nav {
    width: 100%;
    overflow-x: auto;
    padding-bottom: 2px;
  }

  .hero {
    padding-top: 30px;
  }

  h1 {
    font-size: 28px;
  }

  .subject-grid {
    grid-template-columns: 1fr;
  }

  .panel-head,
  .question-row {
    grid-template-columns: 1fr;
    flex-direction: column;
    align-items: stretch;
  }

  .question-row {
    display: block;
  }

  .row-side {
    margin-top: 10px;
    text-align: left;
  }

  .article {
    padding: 0;
  }

  .question-card {
    padding: 0 0 24px;
  }

  .article-guide {
    position: static;
    margin: 24px 0 0;
  }

  .answer-box {
    margin: 24px 0 0;
  }

  .stem {
    font-size: 18px;
  }

  .article-section {
    margin-left: 0;
    margin-right: 0;
  }

  .pager {
    grid-template-columns: 1fr;
  }

  .pager a:last-child {
    text-align: left;
  }

  .compare-table,
  .compare-table thead,
  .compare-table tbody,
  .compare-table tr,
  .compare-table th,
  .compare-table td {
    display: block;
  }

  .compare-table {
    min-width: 0;
    border-collapse: separate;
    border-spacing: 0 10px;
  }

  .compare-table thead {
    display: none;
  }

  .compare-table tr {
    border: 1px solid var(--gray-150);
    border-radius: var(--radius-md);
    overflow: hidden;
    background: var(--white);
  }

  .compare-table td {
    border: 0;
    border-bottom: 1px solid var(--gray-150);
  }

  .compare-table td:last-child {
    border-bottom: 0;
  }
}