/* 老师接力条。只在老师登录且选了「本节课在给谁上课」时才出现，学生端不渲染。
   视觉上刻意做得低调：它是给老师看的旁注，不该抢学生练习内容的位置。
   全站规范：无阴影、图标一律内联 SVG。 */
.rb {
  margin: 12px 0 0;
  border: 1px solid #dbeafe;
  background: #f8fbff;
  border-radius: 12px;
  overflow: hidden;
}
.rb-head {
  display: flex; align-items: center; gap: 9px;
  width: 100%; padding: 9px 13px;
  border: 0; background: none; cursor: pointer;
  font: inherit; text-align: left; color: #1e3a5f;
}
.rb-head:hover { background: #f0f7ff; }
.rb-tag {
  flex: none; font-size: 10.5px; font-weight: 800; letter-spacing: .05em;
  color: #1d4ed8; background: #dbeafe; border-radius: 99px; padding: 2px 9px;
}
.rb-sum {
  flex: 1 1 auto; min-width: 0; font-size: 12.5px; line-height: 1.6;
  overflow: hidden; text-overflow: ellipsis; white-space: nowrap;
}
.rb-sum b { color: #b91c1c; }
.rb-chev { flex: none; opacity: .6; transition: transform .16s; }
.rb-head.on .rb-chev { transform: rotate(180deg); }
/* 展开后摘要不再省略，长内容换行显示 */
.rb-head.on .rb-sum { white-space: normal; overflow: visible; }

.rb-body { padding: 2px 13px 12px; border-top: 1px solid #e8f1fb; }
.rb-sec { margin-top: 10px; }
.rb-k {
  display: block; font-size: 10.5px; font-weight: 800; letter-spacing: .05em; color: #64748b;
}
.rb-k i { font-style: normal; font-weight: 600; opacity: .8; }
.rb-ul { margin: 5px 0 0; padding-left: 17px; font-size: 12.5px; line-height: 1.85; color: #334155; }
.rb-ul b { color: #b91c1c; }
.rb-none { display: block; margin-top: 4px; font-size: 12.5px; color: #94a3b8; }

@media (max-width: 640px) {
  .rb-sum { white-space: normal; }
}