:root{--bg:#f5f7fb;--surface:#fff;--ink:#13213a;--sub:#596981;--muted:#8894a7;--line:#e3e8f0;--brand:#3658db;--brand-dark:#2645b7;--brand-soft:#eef2ff;--green:#138361;--green-soft:#e6f7f1;--red:#bd4d58;--red-soft:#fff0f1;--gold:#a56b16;--radius:18px;color-scheme:light;font-synthesis:none}
*{box-sizing:border-box}html{scroll-behavior:smooth}body{margin:0;font-family:Inter,-apple-system,BlinkMacSystemFont,"Segoe UI",Arial,sans-serif;background:var(--bg);color:var(--ink);line-height:1.58;-webkit-font-smoothing:antialiased}button,input,select,textarea{font:inherit}button{cursor:pointer}a{color:var(--brand);text-decoration:none}a:hover{text-decoration:underline}button:focus-visible,a:focus-visible,input:focus-visible,select:focus-visible,textarea:focus-visible{outline:3px solid #87a3ff;outline-offset:3px}
.site-header{height:80px;background:#111d34;color:white;display:flex;justify-content:space-between;align-items:center;padding:0 max(24px,calc((100vw - 1340px)/2));gap:25px}.brand{display:flex;align-items:center;gap:13px;white-space:nowrap}.brand-logo{display:grid;place-items:center;width:42px;height:42px;border:1px solid #546386;border-radius:12px;font-weight:800;font-size:27px;letter-spacing:-2px;background:#1d2d52}.brand-dot{color:#a5b8ff}.brand strong{display:block;font-size:17px;letter-spacing:-.4px;line-height:1.3}.brand-light{font-weight:400;color:#b9c8e6}.brand small{font-size:10px;color:#9baecc;letter-spacing:.03em;display:block;margin-top:3px}.site-header nav{display:flex;align-items:center;gap:8px}.site-header nav a{padding:9px 16px;color:#bbc9e1;border-radius:10px;font-size:13px;font-weight:650;white-space:nowrap}.site-header nav a:hover,.site-header nav a.nav-active{background:#293958;color:white;text-decoration:none}
.main-wrap{max-width:1390px;margin:auto;padding:36px 24px 58px}.page-intro{display:flex;justify-content:space-between;align-items:end;gap:24px;margin:0 0 27px}.eyebrow{font-weight:800;color:var(--brand);font-size:11px;letter-spacing:.18em;margin:0 0 12px}.page-intro h1{font-size:clamp(28px,3.4vw,44px);line-height:1.14;margin:0;letter-spacing:-1.7px;font-weight:780}.page-intro h1 span{color:#62748d}.intro-copy{color:var(--sub);font-size:13.5px;margin:14px 0 0;max-width:620px}.intro-pill{background:#e9eef9;color:#486288;border-radius:999px;font-size:11px;font-weight:600;padding:10px 13px;white-space:nowrap;margin-bottom:3px}.live-circle{display:inline-block;background:#26b78a;width:7px;height:7px;border-radius:50%;margin-right:4px}.stats{display:grid;grid-template-columns:repeat(4,minmax(0,1fr));gap:14px;margin:0 0 20px}.stat{background:var(--surface);border:1px solid var(--line);border-radius:var(--radius);padding:20px 22px;min-height:126px;box-shadow:0 3px 12px rgba(27,48,89,.025)}.stat>span{display:block;color:#758299;font-weight:790;letter-spacing:.075em;font-size:10px}.stat b{display:block;font-size:31px;line-height:1.25;margin:10px 0 1px;letter-spacing:-1.2px;font-variant-numeric:tabular-nums}.stat small{color:#8793a6;font-size:11px}
.workspace{display:grid;grid-template-columns:326px minmax(0,1fr);gap:17px;align-items:start}.sidebar,.question-panel{background:#fff;border:1px solid var(--line);border-radius:var(--radius);box-shadow:0 4px 18px rgba(27,48,89,.025)}.sidebar{padding:22px 16px 14px;position:sticky;top:14px}.sidebar-head{display:flex;justify-content:space-between;align-items:center;gap:10px;margin:0 7px 20px}.sidebar-head h2{font-size:16px;letter-spacing:-.4px;margin:0 0 3px}.sidebar-head p{font-size:11px;color:var(--muted);margin:0}.icon-button{width:31px;height:31px;border:1px solid var(--line);border-radius:9px;background:#f9fafc;color:var(--sub);font-size:18px}.filter-group{margin:0 6px 13px}.filter-group label{display:block;font-size:11px;color:#68778c;font-weight:720;margin:0 0 6px}.filter-group select,.filter-group input{width:100%;border:1px solid #dce3ed;border-radius:10px;padding:10px 11px;background:#fff;color:var(--ink);font-size:12px;min-height:39px;outline-offset:2px}.filter-group input::placeholder{color:#9ba7ba}.filter-grid{display:grid;grid-template-columns:1fr 1fr;gap:6px}.question-list{border-top:1px solid #ebeff5;padding:11px 4px 5px;margin-top:15px;max-height:502px;overflow:auto;scrollbar-width:thin}.question-link{width:100%;display:flex;align-items:flex-start;gap:10px;border:1px solid transparent;border-radius:11px;background:transparent;text-align:left;padding:10px 9px;margin:2px 0;color:var(--ink)}.question-link:hover{background:#f5f7fd}.question-link.active{border-color:#cfd9ff;background:#eff3ff}.question-link .qid{flex:0 0 auto;background:#eef2f7;color:#677791;min-width:35px;border-radius:7px;padding:4px;text-align:center;font-size:10px;font-weight:800}.question-link.active .qid{background:#dce6ff;color:var(--brand)}.question-link .ql-info{min-width:0;flex:1}.ql-info strong{display:block;font-size:12px;font-weight:700;white-space:nowrap;text-overflow:ellipsis;overflow:hidden}.ql-info small{display:block;color:#8996a8;font-size:10px;white-space:nowrap;text-overflow:ellipsis;overflow:hidden;margin-top:3px}.marker{font-size:15px;font-weight:800;line-height:1.4;color:#c3cbd5}.marker.right{color:var(--green)}.marker.wrong{color:var(--red)}.marker.review{color:var(--gold)}.side-footer{color:#97a1b2;font-size:10px;margin:14px 9px 0;border-top:1px solid #eef1f5;padding-top:13px}.hint{padding:10px;font-size:12px;color:var(--muted)}
.question-panel{min-height:680px;overflow:hidden}.question-top{display:flex;align-items:center;justify-content:space-between;gap:12px;padding:20px 30px 16px;border-bottom:1px solid #eef1f5}.crumb{font-size:11px;color:#728099;font-weight:700}.count-chip{font-size:11px;color:var(--brand);background:#eef2ff;border:1px solid #dce5ff;border-radius:99px;padding:6px 10px;font-variant-numeric:tabular-nums;font-weight:750}.progress-track{height:3px;background:#f0f2f8;width:100%}.progress-fill{height:100%;width:0;background:linear-gradient(90deg,#4069e6,#88a2f8);transition:width .2s}.question-content{padding:29px 34px 34px;min-height:520px}.question-heading{margin:0 0 18px}.badges{display:flex;flex-wrap:wrap;gap:7px;margin-bottom:15px}.tag{display:inline-flex;align-items:center;border-radius:7px;background:#f1f4f8;color:#65758e;padding:5px 8px;font-size:10px;font-weight:750}.tag-blue{background:#ecf1ff;color:#3658db}.tag-orange{background:#fff3e4;color:#a66a1c}.question-heading h2{font-size:clamp(21px,2.15vw,29px);margin:0;line-height:1.27;letter-spacing:-.9px}.question-id{color:#99a4b5;font-size:11px;font-weight:700;margin:9px 0 0}.markdown-content{font-size:14px;line-height:1.85;color:#2e3e56;overflow-wrap:anywhere}.markdown-content h1,.markdown-content h2,.markdown-content h3{color:#162846;line-height:1.3;letter-spacing:-.4px;margin:1.5em 0 .7em}.markdown-content h2{font-size:19px}.markdown-content h3{font-size:16px}.markdown-content p{margin:0 0 13px}.markdown-content strong{color:#1d2e4d}.markdown-content pre{background:#111c30;color:#e4edfa;border-radius:12px;overflow:auto;padding:17px 19px;font-size:12px;line-height:1.6;white-space:pre}.markdown-content code{background:#eff2f7;color:#314b80;border:1px solid #e6eaf2;border-radius:5px;padding:2px 5px;font-family:ui-monospace,SFMono-Regular,Menlo,Consolas,monospace;font-size:.88em}.markdown-content pre code{background:transparent;color:inherit;padding:0;border:0}.markdown-content blockquote{border-left:3px solid #a7bafa;padding:10px 15px;margin:10px 0 16px;background:#f6f8ff;border-radius:0 9px 9px 0}.markdown-content blockquote p:last-child{margin-bottom:0}.markdown-content ul,.markdown-content ol{padding-left:22px}.markdown-content li{margin:5px 0}.markdown-content table{border-collapse:collapse;width:max-content;min-width:100%;font-size:12px;line-height:1.5}.markdown-content th,.markdown-content td{text-align:left;border:1px solid #dde4ef;padding:8px 11px}.markdown-content th{background:#f1f5fb}.markdown-content table{display:block;max-width:100%;overflow:auto}.markdown-content hr{border:none;border-top:1px solid var(--line);margin:23px 0}.markdown-content img{max-width:100%}.markdown-content a{border-bottom:1px solid #ccd7ff}.divider-label{border-top:1px solid #e8edf4;margin:27px 0 17px;padding-top:23px;font-size:11px;font-weight:800;color:#7c899c;text-transform:uppercase;letter-spacing:.1em}
.answers{display:grid;gap:9px;margin:19px 0 16px}.answer-button{width:100%;display:flex;gap:13px;align-items:flex-start;border:1px solid #e0e6ef;border-radius:12px;background:#fff;color:#26354e;text-align:left;padding:13px 15px;transition:background .12s,border-color .12s}.answer-button .option-letter{border:1px solid #dce3ef;border-radius:8px;background:#f5f7fb;min-width:29px;height:29px;display:grid;place-items:center;font-size:12px;font-weight:800}.answer-button .option-text{font-size:13px;line-height:1.6;padding-top:3px;overflow-wrap:anywhere;min-width:0}.answer-button .option-text code{font-size:.91em;background:#f3f5f8;padding:1px 3px;border-radius:3px}.answer-button:hover:not(:disabled){border-color:#9db2ff;background:#f7f9ff}.answer-button:disabled{cursor:default}.answer-button.chosen{border-color:#496bdb;background:#f1f5ff}.answer-button.is-right{border-color:#77cdb6;background:#ebfaf5}.answer-button.is-wrong{border-color:#eca9ae;background:#fff2f2}.answer-button.is-right .option-letter{background:#caeddf;color:#0c7859;border-color:transparent}.answer-button.is-wrong .option-letter{background:#ffdbdd;color:#aa3d4a;border-color:transparent}
.written-input{width:100%;border:1px solid #dbe3ef;border-radius:12px;background:#fff;min-height:120px;resize:vertical;padding:15px;font-size:13px;line-height:1.65;color:var(--ink)}.written-input::placeholder{color:#9aa6b8}.answer-action{display:flex;align-items:center;gap:10px;flex-wrap:wrap;margin:13px 0 10px}.btn{border:1px solid transparent;border-radius:10px;padding:11px 17px;font-size:12px;font-weight:740;transition:background .1s}.btn-primary{background:var(--brand);color:white}.btn-primary:hover:not(:disabled){background:var(--brand-dark)}.btn-subtle{background:#fff;border-color:#dfe5ee;color:#51617b}.btn-subtle:hover:not(:disabled){background:#f2f5fc}.btn:disabled{opacity:.45;cursor:not-allowed}.tiny-note{font-size:11px;color:#8d99a9}.solution-box{background:#f7f9fe;border:1px solid #e0e8f7;border-radius:14px;margin:20px 0 6px;padding:20px 22px;animation:fadeIn .18s ease}.solution-top{display:flex;justify-content:space-between;align-items:center;gap:10px;margin-bottom:14px}.solution-top h3{font-size:15px;letter-spacing:-.3px;margin:0}.solution-status{font-size:11px;font-weight:700;padding:5px 9px;border-radius:8px;background:#ebeffb;color:#3658db}.solution-status.right{background:var(--green-soft);color:var(--green)}.solution-status.wrong{background:var(--red-soft);color:var(--red)}.solution-box .markdown-content{font-size:13px}.solution-box .markdown-content h1:first-child{display:none}.solution-box .markdown-content h2:first-child{margin-top:0}.solution-box .markdown-content h2{font-size:16px}.self-grade{display:flex;gap:9px;align-items:center;flex-wrap:wrap;border-top:1px solid #dde5f1;margin-top:17px;padding-top:15px}.self-grade span{font-size:12px;color:#65758d;margin-right:auto}.grade-btn{padding:9px 11px;background:#fff;border:1px solid #d8e0ed;border-radius:9px;font-size:12px;font-weight:750}.grade-btn.green{color:#147c5f}.grade-btn.red{color:#b54452}.grade-btn:hover{background:#f5f7fc}.grade-btn.selected{background:#e7eefc;border-color:#7f98e1}.issue-warning{padding:12px 14px;border:1px solid #f0d7ab;background:#fff9eb;border-radius:10px;color:#815b21;font-size:12px;line-height:1.6;margin:14px 0}.related{display:flex;align-items:center;justify-content:space-between;gap:12px;padding:15px 17px;background:#f9faff;border:1px solid #e9ecf7;border-radius:11px;margin-top:23px;flex-wrap:wrap}.related small{font-size:10px;color:#8a97ad;display:block}.related strong{font-size:12px}.related a{font-size:12px;font-weight:700}.question-nav{padding:18px 30px;border-top:1px solid #edf0f5;display:flex;align-items:center;justify-content:space-between;gap:14px}.keyboard-hint{font-size:10px;color:#9da7b7}.empty-state{text-align:center;padding:70px 20px;color:#7e8ba0}.empty-state h2{color:#273c59;font-size:19px;letter-spacing:-.4px}.empty-state p{font-size:13px}.loader{margin:auto;width:25px;height:25px;border-radius:50%;border:3px solid #e5ebf8;border-top-color:var(--brand);animation:spin .6s linear infinite}.source-disclaimer{font-size:11px;color:#9ba4b4;margin:25px auto;text-align:center;max-width:940px}
.lesson-intro{margin-bottom:24px}.notes-workspace .question-list{max-height:570px}.reading-panel{min-height:750px}.reading-content{padding:28px 35px 55px;min-height:680px}.reading-content>h1{font-size:clamp(25px,2.5vw,34px);line-height:1.2;letter-spacing:-1px;margin:0 0 11px}.reading-content .note-subtitle{font-size:12px;color:#8190a6;margin:0 0 30px;padding-bottom:18px;border-bottom:1px solid #ecf0f5}.reading-content .markdown-content{max-width:820px;font-size:14px}.reading-content .markdown-content h2{margin-top:2em}.text-link{font-size:12px;font-weight:700}.note-chapter{font-size:11px;color:#4561b6;background:#edf2ff;border-radius:7px;padding:5px 9px;display:inline-block;margin-bottom:12px}
@keyframes spin{to{transform:rotate(360deg)}}@keyframes fadeIn{from{opacity:0;transform:translateY(5px)}to{opacity:1;transform:translateY(0)}}
@media(max-width:980px){.workspace{grid-template-columns:275px minmax(0,1fr)}.sidebar{padding:18px 12px}.question-content{padding:25px 22px}.question-top,.question-nav{padding-left:22px;padding-right:22px}.stats{gap:9px}.stat{padding:16px}.intro-pill{display:none}}
@media(max-width:720px){.site-header{height:auto;min-height:71px;padding:12px 17px;flex-wrap:wrap;gap:8px}.site-header nav{width:100%;justify-content:space-between}.site-header nav a{flex:1;text-align:center}.main-wrap{padding:25px 12px 35px}.page-intro{padding:0 6px}.page-intro h1{letter-spacing:-1px}.intro-copy{font-size:12px}.stats{grid-template-columns:repeat(2,minmax(0,1fr));gap:9px}.stat{min-height:100px;padding:13px 15px;border-radius:13px}.stat b{font-size:25px;margin:5px 0}.stat small{font-size:10px}.workspace{grid-template-columns:minmax(0,1fr);gap:12px}.sidebar{position:static;border-radius:14px;padding:18px 12px 9px}.question-list{max-height:195px}.question-panel{min-height:580px;border-radius:14px}.question-top{padding:15px 17px}.question-content{padding:22px 17px 30px;min-height:425px}.question-nav{padding:14px 17px}.keyboard-hint{display:none}.btn{padding:10px 13px}.answer-button{padding:11px 10px;gap:9px}.reading-content{padding:23px 18px 40px}.reading-panel{min-height:580px}.related{padding:12px}}

/* Four-option test additions */
.test-instruction { border-left: 3px solid #4c80e9; background: rgba(78,128,225,.07); color: var(--text, #17223a); font-size: .88rem; margin: 1rem 0; padding: .75rem 1rem; border-radius: .3rem; }
.answer-result { margin: .3rem 0 1.2rem; padding: .85rem 1rem; border-radius: .6rem; background: rgba(51, 154, 107, .09); color: var(--text, #17223a); overflow-wrap: anywhere; line-height: 1.65; }
.answer-button .option-text { white-space: pre-wrap; overflow-wrap: anywhere; }