.study-article{--sa-bg:#fffaf0;--sa-paper:#fffdf5;--sa-ink:#0c2457;--sa-muted:#35527a;--sa-line:#a9cfe6;--sa-green:#246331;--sa-soft:#edf8e9;--sa-tip:#e5f4ff;--sa-orange:#d9432b;--sa-blue:#095994;--sa-garden:url(/guides/garden-day-wide-v5.avif);--sa-hand:"Kalam", "Ma Shan Zheng", cursive;--sa-gutter:clamp(28px, 5vw, 78px);isolation:isolate;min-height:100vh;color:var(--sa-ink);background:var(--sa-bg);--lightningcss-light:initial;--lightningcss-dark: ;color-scheme:light;padding-bottom:48px;font:17px/1.8 "Source Sans 3 Variable",PingFang SC,Microsoft YaHei,sans-serif;position:relative}.study-article:lang(zh-CN){--sa-hand:"Ma Shan Zheng", "Kalam", cursive}.study-article[data-article-theme=dark]{--sa-bg:#05213d;--sa-paper:#1e364d;--sa-ink:#fff3d9;--sa-muted:#c2d4e7;--sa-line:#50759a;--sa-green:#c0e9c1;--sa-soft:#153d34;--sa-tip:#14385b;--sa-orange:#ff9b84;--sa-blue:#b5ddff;--sa-garden:url(/guides/garden-night-wide-v5.avif);--lightningcss-light: ;--lightningcss-dark:initial;color-scheme:dark}@media (prefers-color-scheme:dark){.study-article:not([data-article-theme=light]){--sa-bg:#05213d;--sa-paper:#1e364d;--sa-ink:#fff3d9;--sa-muted:#c2d4e7;--sa-line:#50759a;--sa-green:#c0e9c1;--sa-soft:#153d34;--sa-tip:#14385b;--sa-orange:#ff9b84;--sa-blue:#b5ddff;--sa-garden:url(/guides/garden-night-wide-v5.avif);--lightningcss-light: ;--lightningcss-dark:initial;color-scheme:dark}}.study-article *{box-sizing:border-box}.study-article :is(h1,h2,h3,p,ul,ol,dl,figure){margin-top:0}.study-article :is(h1,h2,h3){color:inherit;letter-spacing:0;font-family:inherit;font-weight:700;line-height:1.45}.study-article a{color:var(--sa-blue);text-underline-offset:4px}.study-article :is(button,input){font:inherit}.study-article button{cursor:pointer}.study-article :is(a,button,input,summary):focus-visible{outline-offset:4px;outline:3px solid #d58b1e}.sa-sky{z-index:-1;pointer-events:none;background:var(--sa-garden) center top / max(1600px,100%) auto no-repeat;height:150px;position:absolute;inset:0 0 auto;-webkit-mask-image:linear-gradient(#000 20%,#0000);mask-image:linear-gradient(#000 20%,#0000)}.sa-skip{z-index:30;background:var(--sa-paper);padding:10px 18px;position:absolute;top:8px;left:20px;transform:translateY(-200%)}.sa-skip:focus{transform:none}.sa-header,.sa-main,.sa-footer{width:min(1200px,calc(100% - 2 * var(--sa-gutter)));margin-inline:auto;position:relative}.sa-header{z-index:10;justify-content:space-between;align-items:center;gap:24px;min-height:80px;display:flex}.sa-header>a{flex-shrink:0;text-decoration:none;display:inline-flex}.sa-header .brand-lockup{align-items:center;gap:10px;display:inline-flex}.sa-header .brand-lockup__mark{width:46px;height:46px}.sa-header .brand-lockup__copy strong{letter-spacing:-.6px;color:var(--sa-ink);font-size:30px}.sa-navigation{border:1px solid color-mix(in srgb,var(--sa-line) 60%,transparent);background:color-mix(in srgb,var(--sa-paper) 91%,transparent);border-radius:40px;align-items:center;gap:22px;padding:2px 14px 2px 24px;font-size:16px;font-weight:600;line-height:1.45;display:flex}.sa-navigation>a,.sa-subject-menu>summary{white-space:nowrap;min-height:44px;color:var(--sa-ink);align-items:center;gap:9px;text-decoration:none;display:inline-flex}.sa-navigation>a:hover,.sa-subject-menu>summary:hover{color:var(--sa-orange)}.sa-subject-menu{position:relative}.sa-subject-menu>summary{cursor:pointer;list-style:none}.sa-subject-menu>summary::-webkit-details-marker{display:none}.sa-subject-menu>summary>span{font:22px/1 serif}.sa-subject-menu ul{border:1px solid var(--sa-line);background:var(--sa-paper);border-radius:9px;width:250px;margin:0;padding:8px;list-style:none;position:absolute;top:calc(100% + 9px);right:-24px;box-shadow:0 12px 30px #061a3d30}.sa-subject-menu li+li{border-top:1px dashed var(--sa-line)}.sa-subject-menu li a{justify-content:space-between;align-items:center;min-height:48px;padding:10px;font-size:16px;text-decoration:none;display:flex}.sa-subject-menu li a:hover{background:var(--sa-tip)}.sa-theme{background:color-mix(in srgb,var(--sa-line) 28%,transparent);border-radius:30px;flex-shrink:0;gap:2px;padding:3px;display:flex}.sa-theme button{width:38px;height:38px;color:var(--sa-blue);background:0 0;border:0;border-radius:25px;flex:0 0 38px;place-items:center;padding:8px;line-height:1;display:grid}.sa-theme button:first-child{color:#ba7910}.sa-theme button[aria-pressed=true]{background:var(--sa-paper);box-shadow:0 1px 4px #0e2b4b25}.sa-theme svg{width:20px;height:20px;display:block}.sa-mobile-menu{display:none}.sa-breadcrumb{color:var(--sa-muted);flex-wrap:wrap;gap:12px;margin-bottom:22px;padding-top:28px;font-size:14px;display:flex}.sa-breadcrumb a{text-decoration:none}.sa-hero{grid-template-columns:minmax(0,1.15fr) minmax(0,.85fr);align-items:center;gap:32px;padding-bottom:38px;display:grid}.sa-hero-copy{min-width:0}.sa-kicker{color:var(--sa-blue);letter-spacing:.12em;font-size:11px;font-weight:700;margin-bottom:10px!important}.sa-hero .sa-kicker{background:var(--sa-soft);color:var(--sa-green);border-radius:3px 7px 2px 5px;padding:5px 11px;display:inline-block}.study-article .sa-hero h1{font:400 clamp(39px,4.2vw,61px)/1.23 var(--sa-hand);letter-spacing:0;text-wrap:balance;margin:7px 0 22px;position:relative}.sa-hero h1:after{content:"";background:var(--sa-orange);border-radius:70%;width:90%;height:4px;position:absolute;bottom:-10px;left:0;transform:rotate(-.8deg)}.sa-description{color:var(--sa-muted);margin:26px 0 18px;font-size:18px;line-height:1.75}.sa-meta{color:var(--sa-muted);flex-wrap:wrap;gap:4px 18px;font-size:12px;display:flex}.sa-hero-art{object-fit:contain;filter:drop-shadow(0 5px 5px #14245315);width:100%;height:330px}.sa-layout{grid-template-columns:180px minmax(0,1fr);align-items:start;gap:28px;margin-bottom:48px;display:grid}.sa-toc{align-self:start;padding:26px 8px 0 4px;font-size:14px;position:sticky;top:25px}.sa-toc>p{font:400 26px/1.3 var(--sa-hand);margin-bottom:22px}.sa-toc nav{gap:14px;display:grid}.sa-toc nav a{min-height:44px;color:var(--sa-muted);gap:12px;line-height:1.65;text-decoration:none;display:flex}.sa-toc nav a:hover{color:var(--sa-blue);text-decoration:underline}.sa-toc nav span{color:var(--sa-orange);flex-shrink:0;padding-top:4px;font-size:11px}.sa-toc-back{margin-top:34px;font-size:13px;display:block}.sa-paper{isolation:isolate;filter:drop-shadow(0 4px 4px #6a491b12);min-width:0;padding:12px 42px 28px;position:relative}.sa-paper:before{content:"";z-index:-1;background:var(--sa-paper);clip-path:polygon(.6% .2%,13% 0,29% .15%,43% 0,58% .2%,74% 0,90% .15%,99% .2%,100% 2%,99.7% 31%,100% 60%,99.6% 92%,99% 99.8%,80% 100%,58% 99.85%,36% 100%,12% 99.8%,.6% 100%,0 98%,.4% 65%,0 36%,.4% 2%);position:absolute;inset:0}.sa-animal{align-items:center;gap:18px;padding:24px 0 10px;display:flex}.sa-animal img{object-fit:contain;width:100px;height:84px}.sa-animal p{color:var(--sa-green);margin:0;font-size:14px;line-height:1.7}.sa-animal span{letter-spacing:.05em;margin-bottom:4px;font-size:11px;font-weight:700;display:block}.sa-prose>section{border-bottom:1px solid var(--sa-line);padding:33px 0;scroll-margin-top:25px}.sa-prose>section:last-child{border:0}.sa-prose h2{margin:0 0 19px;font-size:24px}.sa-prose h3{margin-bottom:10px;font-size:18px}.sa-prose p{margin-bottom:18px}.sa-prose p:last-child{margin-bottom:0}.sa-answer{border-left:3px solid var(--sa-green);background:var(--sa-soft);padding:15px 20px;font-size:18px;line-height:1.95}.sa-prose li{margin-bottom:10px;padding-left:4px}.sa-prose ol,.sa-prose ul{padding-left:24px}.sa-callout{background:var(--sa-soft);border-radius:5px;margin:22px 0;padding:19px 22px;font-size:15px}.sa-callout strong{color:var(--sa-green)}.sa-equation{color:var(--sa-green);text-align:center;background:var(--sa-soft);border:1px solid var(--sa-line);border-radius:5px;padding:25px 8px;font-size:23px;overflow-x:auto}.sa-table{margin:22px 0;overflow:auto}.sa-table table{border-collapse:collapse;width:100%;font-size:15px}.sa-table caption{text-align:left;color:var(--sa-muted);margin-bottom:8px;font-size:12px}.sa-table :is(th,td){text-align:left;border-bottom:1px solid var(--sa-line);min-width:80px;padding:12px}.sa-table th{color:var(--sa-green);background:var(--sa-soft);font-weight:650}.sa-figure{margin:24px 0}.sa-figure svg{width:100%;height:auto;color:var(--sa-ink);font:15px "Source Sans 3 Variable",sans-serif;display:block}.sa-figure figcaption{color:var(--sa-muted);margin-top:12px;font-size:12px}.sa-errors{gap:15px;display:grid}.sa-errors>div{border-bottom:1px dashed var(--sa-line);padding:0 0 15px}.sa-errors>div:last-child{border:0;padding:0}.sa-errors strong{color:var(--sa-orange)}.sa-errors p{margin:4px 0 0;font-size:16px}.sa-next{flex-wrap:wrap;gap:12px;margin-top:22px;display:flex}.study-article .sa-button{border:1px solid var(--sa-green);background:var(--sa-green);min-height:45px;color:var(--sa-paper);border-radius:5px;justify-content:center;align-items:center;padding:10px 19px;font-size:15px;font-weight:650;text-decoration:none;display:inline-flex}.sa-button:disabled{opacity:.5;cursor:default}.sa-next>a:not(.sa-button){align-self:center;font-size:14px}.sa-mobile-toc{display:none}.sa-sources{color:var(--sa-muted);font-size:13px}.sa-sources h2{font-size:19px}.sa-sources li p{margin:4px 0;font-size:12px}.sa-review-note{border-top:1px dashed var(--sa-line);padding-top:14px;font-size:12px}.sa-exercise{background:var(--sa-bg);border:1px solid var(--sa-line);border-radius:7px;padding:24px}.sa-exercise-label{color:var(--sa-orange);letter-spacing:.06em;font-size:11px;font-weight:700}.sa-stem{margin:20px 0;font-size:16px;font-weight:600}.sa-exercise fieldset{border:0;margin:20px 0;padding:0}.sa-exercise legend{color:var(--sa-muted);margin-bottom:8px;font-size:12px}.sa-option{border:1px solid var(--sa-line);cursor:pointer;border-radius:4px;align-items:baseline;gap:12px;margin-top:8px;padding:12px;font-size:15px;line-height:1.6;display:flex}.sa-option.is-selected{border-color:var(--sa-green);background:var(--sa-soft)}.sa-option input{accent-color:var(--sa-green)}.sa-exercise-actions{flex-wrap:wrap;justify-content:space-between;gap:12px;display:flex}.sa-exercise-actions>button:not(.sa-button){color:var(--sa-blue);background:0 0;border:0;padding:10px 0;font-size:14px}.sa-exercise-note{color:var(--sa-muted);margin:15px 0 0!important;font-size:11px!important}.sa-exercise img{max-width:100%;height:auto}.sa-dialog{border:1px solid var(--sa-line);background:var(--sa-paper);width:min(620px,100vw - 32px);max-height:80dvh;color:var(--sa-ink);border-radius:9px;margin:auto;padding:24px 30px;font-family:inherit;line-height:1.8;position:fixed;inset:0;overflow:auto;box-shadow:0 24px 100px #001a3b55}.sa-dialog::backdrop{background:#071d3c88}.sa-dialog-head{border-bottom:1px solid var(--sa-line);justify-content:space-between;align-items:center;margin-bottom:20px;display:flex}.sa-dialog-head h3{font-size:20px}.sa-dialog-head button{min-width:44px;height:44px;color:var(--sa-ink);background:0 0;border:0;align-self:start}.sa-dialog .sa-verdict{color:var(--sa-green);font-weight:700}.sa-dialog dl>div{border-top:1px dashed var(--sa-line);padding:12px 0;font-size:14px}.sa-dialog dt{font-weight:700}.sa-dialog dd{margin:3px 0}.sa-dialog .sa-button{margin-top:15px}.study-article p{font-weight:400;line-height:1.85}.study-article .sa-prose h2{font:400 34px/1.35 var(--sa-hand);margin-bottom:24px;display:inline-block;position:relative}.sa-prose h2:after{content:"";background:var(--sa-orange);border-radius:65%;height:2px;position:absolute;bottom:-7px;left:0;right:0;transform:rotate(-.6deg)}.sa-prose>section{border-bottom:1px dashed var(--sa-line);padding:32px 0}.sa-answer{background:var(--sa-tip);border:0;border-radius:7px 13px 6px 10px;padding:20px 23px}.sa-callout{border-radius:5px 12px 4px 9px}.sa-animal{padding:22px 0 8px}.sa-animal img{width:78px;height:78px}.sa-animal p{color:var(--sa-muted);font-size:14px}.sa-animal span{color:var(--sa-green);font-size:13px}.sa-figure{background:color-mix(in srgb,var(--sa-tip) 35%,var(--sa-paper));border:1px solid var(--sa-line);border-radius:4px 9px 4px 6px;padding:18px}.sa-figure svg{color:var(--sa-ink)}.sa-equation{background:var(--sa-tip);color:var(--sa-blue)}.sa-exercise{background:var(--sa-soft);border:0;border-radius:5px 13px 4px 9px}.sa-option{background:var(--sa-paper)}.sa-option.is-selected{background:var(--sa-tip);border-color:var(--sa-blue)}.study-article .sa-button{background:var(--sa-blue);color:var(--sa-bg);border:0;border-radius:8% 6% 10% 7%/18% 16% 20%}.sa-sources,.sa-sources li p{font-size:13px}.sa-sources .sa-review-note{font-size:12px}.sa-footer{background:var(--sa-paper);z-index:1;border:0;border-radius:7px 24px 10px 19px;flex-wrap:wrap;align-items:center;gap:20px;padding:17px 25px;display:flex}.sa-footer-brand{flex-shrink:0;text-decoration:none;display:inline-flex}.sa-footer .brand-lockup{align-items:center;gap:8px;display:inline-flex}.sa-footer .brand-lockup__mark{width:30px;height:30px}.sa-footer .brand-lockup__copy strong{color:var(--sa-ink);font-size:22px}.sa-footer nav{flex-wrap:wrap;gap:16px;font-size:12px;display:flex}.sa-footer nav a{align-items:center;min-height:44px;text-decoration:none;display:inline-flex}.sa-footer nav a:hover{text-decoration:underline}.sa-footer>p{color:var(--sa-muted);margin:0 0 0 auto;font-size:11px}@media (min-width:2400px){.study-article{--sa-gutter:100px}.sa-header,.sa-main,.sa-footer{width:1680px}.sa-layout{grid-template-columns:240px minmax(0,1fr);gap:44px}.sa-paper{padding-inline:90px}.sa-prose{font-size:20px}.study-article .sa-hero h1{font-size:76px}.sa-hero-art{height:420px}}@media (max-width:1150px){.sa-navigation{gap:14px;padding-left:18px;font-size:14px}.sa-header .brand-lockup__copy strong{font-size:26px}.sa-layout{grid-template-columns:150px minmax(0,1fr);gap:22px}.sa-paper{padding-inline:32px}}@media (max-width:950px){.sa-desktop-nav{display:none}.sa-mobile-menu{display:block;position:relative}.sa-mobile-menu>summary{background:var(--sa-paper);cursor:pointer;border-radius:6px;align-content:center;gap:5px;width:44px;height:44px;padding:10px;list-style:none;display:grid}.sa-mobile-menu>summary::-webkit-details-marker{display:none}.sa-mobile-menu>summary span{background:var(--sa-ink);width:24px;height:2px;display:block}.sa-mobile-menu>.sa-navigation{background:var(--sa-paper);border-radius:10px;gap:3px;width:300px;max-width:calc(100vw - 56px);padding:13px;display:grid;position:absolute;top:calc(100% + 10px);right:0;box-shadow:0 12px 30px #061a3d40}.sa-mobile-menu .sa-navigation>a,.sa-mobile-menu .sa-subject-menu>summary{padding:6px 10px}.sa-mobile-menu .sa-theme{justify-self:start;margin:6px}.sa-mobile-menu .sa-subject-menu ul{background:var(--sa-tip);width:auto;box-shadow:none;border:0;position:static}}@media (max-width:760px){.sa-sky{height:100px}.study-article{--sa-gutter:18px;padding-bottom:32px}.sa-header{min-height:70px}.sa-header .brand-lockup__copy strong{font-size:25px}.sa-header .brand-lockup__mark{width:40px;height:40px}.sa-breadcrumb{margin-bottom:18px;padding-top:18px;font-size:12px}.sa-hero{flex-direction:column;align-items:stretch;gap:8px;padding-bottom:26px;display:flex}.study-article .sa-hero h1{font-size:43px;line-height:1.25}.sa-hero .sa-kicker{font-size:10px}.sa-description{font-size:16px}.sa-meta{gap:4px 12px;font-size:11px}.sa-hero-art{height:230px}.sa-layout{margin-bottom:28px;display:block}.sa-toc{display:none}.sa-paper{padding:22px 22px 10px}.sa-mobile-toc{margin:0 0 12px;font-size:14px;display:block;position:relative}.sa-mobile-toc summary{border:1px solid var(--sa-line);cursor:pointer;color:var(--sa-blue);border-radius:4px;padding:10px 13px}.sa-mobile-toc nav{z-index:5;background:var(--sa-paper);border:1px solid var(--sa-line);padding:10px;display:grid;position:absolute;top:100%;left:0;right:0;box-shadow:0 10px 20px #001a3b20}.sa-mobile-toc nav a{min-height:44px;padding:9px 12px;text-decoration:none}.sa-prose{font-size:16px}.sa-prose>section{padding:28px 0}.study-article .sa-prose h2{font-size:29px}.sa-prose h3{font-size:18px}.sa-animal{gap:12px;padding-top:10px}.sa-animal img{width:64px;height:64px}.sa-animal p{font-size:13px}.sa-answer{padding:16px 17px;font-size:17px}.sa-figure{padding:10px}.sa-exercise{padding:18px}.sa-option{gap:8px;padding:11px 10px}.sa-dialog{padding:20px}.sa-footer{gap:10px 20px;padding:18px 20px}.sa-footer nav{gap:14px}.sa-footer>p{margin-left:0}}@media (prefers-reduced-motion:reduce){.study-article *{scroll-behavior:auto!important}}.sa-keep{white-space:nowrap}@media (max-width:760px){.sa-header{min-height:62px}.sa-breadcrumb{margin-bottom:12px;padding-top:10px}.sa-hero{grid-template-columns:minmax(0,1fr) 92px;align-items:start;gap:10px;padding-bottom:18px;display:grid}.sa-hero-copy{display:contents}.sa-hero .sa-kicker{grid-column:1/-1;justify-self:start;padding:4px 9px;margin:0!important}.study-article .sa-hero h1{text-wrap:pretty;grid-area:2/1;margin:2px 0 8px;font-size:clamp(28px,7.7vw,34px);line-height:1.22}.sa-hero h1:after{height:3px;bottom:-7px}.sa-hero-art{object-fit:contain;grid-area:2/2;align-self:center;height:100px}.sa-description{grid-column:1/-1;margin:10px 0 2px;font-size:15px;line-height:1.65}.study-article .sa-description{line-height:1.65}.sa-meta{grid-column:1/-1;gap:2px 10px;font-size:10px}.sa-paper{padding-top:16px}.sa-animal{gap:10px;padding:8px 0 0}.sa-animal img{width:48px;height:48px}.sa-animal p{font-size:12px;line-height:1.65}.sa-animal span{font-size:12px}.sa-prose>section:first-child{padding-top:20px}.sa-mobile-toc{margin-bottom:8px}}@media (max-width:360px){.sa-hero{grid-template-columns:minmax(0,1fr) 74px;gap:8px}.sa-hero-art{height:82px}.study-article .sa-hero h1{font-size:28px}}