* {
  box-sizing: border-box;
}

body {
  margin: 0;
  padding: 20px;
  background: #f2f5fa;
  font-family: Arial, Helvetica, sans-serif;
  color: #222;
}

.container {
  max-width: 1000px;
  margin: auto;
}

.main-title {
  text-align: center;
  font-size: 36px;
  color: #0d47a1;
  margin-bottom: 40px;
}

.lesson-block {
  background: white;
  border-radius: 18px;
  padding: 20px;
  margin-bottom: 35px;
  box-shadow: 0 4px 10px rgba(0, 0, 0, 0.08);
}

.sgk-frame {
  border: 1px solid #1976d2;
  border-radius: 12px;
  overflow: hidden;
  background: white;
}

.sgk-label {
  background: #1976d2;
  color: white;
  padding: 10px 14px;
  font-size: 14px;
  font-weight: bold;
}

.sgk-image {
  width: 100%;
  display: block;
}

.page-note {
  margin-top: 15px;
  padding: 12px;
  background: #eef6ff;
  border-left: 5px solid #1976d2;
  border-radius: 8px;
  font-size: 16px;
}

.audio-section {
  margin-top: 20px;
}

.audio-title {
  font-size: 18px;
  font-weight: bold;
  color: #0d47a1;
  margin-bottom: 12px;
}

.button-group {
  display: flex;
  flex-wrap: wrap;
  gap: 10px;
}

button {
  border: none;
  padding: 12px 18px;
  border-radius: 10px;
  cursor: pointer;
  font-size: 15px;
  font-weight: bold;
  transition: 0.2s;
}

button:hover {
  transform: scale(1.03);
}

.btn-audio {
  background: #1976d2;
  color: white;
}

.btn-question {
  background: #ff9800;
  color: white;
}

.btn-answer {
  background: #2e7d32;
  color: white;
}

.btn-stop {
  background: #d32f2f;
  color: white;
}

.text-content-section {
  margin-top: 20px;
  display: flex;
  flex-direction: column;
  gap: 14px;
}

.text-zone {
  padding: 14px 16px;
  border-radius: 10px;
  background: #fafafa;
  border: 1px solid #e0e0e0;
}

.text-zone-label {
  font-size: 15px;
  font-weight: bold;
  margin-bottom: 10px;
}

.text-zone-body {
  font-size: 16px;
  line-height: 1.6;
  min-height: 48px;
}

.text-zone-body p {
  margin: 0 0 8px;
}

.text-zone-body p:last-child {
  margin-bottom: 0;
}

.text-zone-noidung {
  border-left: 5px solid #1976d2;
  background: #eef6ff;
}

.text-zone-noidung .text-zone-label {
  color: #0d47a1;
}

.text-zone-cauhoi {
  border-left: 5px solid #ff9800;
  background: #fff8e1;
}

.text-zone-cauhoi .text-zone-label {
  color: #e65100;
}

.btn-show-answer {
  align-self: flex-start;
  background: #2e7d32;
  color: white;
}

.text-zone-traloi {
  display: none;
  border-left: 5px solid #2e7d32;
  background: #e8f5e9;
}

.text-zone-traloi.is-visible {
  display: block;
}

.text-zone-traloi .text-zone-label {
  color: #1b5e20;
}

@media (max-width: 700px) {
  body {
    padding: 10px;
  }

  .lesson-block {
    padding: 12px;
  }

  button {
    width: 100%;
  }

  .text-zone-body {
    font-size: 15px;
  }
}