* {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        body {
            font-family: 'Segoe UI', Tahoma, Geneva, Verdana, sans-serif;
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            display: flex;
            justify-content: center;
            align-items: center;
            min-height: 100vh;
            padding: 10px;
            overflow: hidden;
        }

        .container {
            background: white;
            border-radius: 20px;
            box-shadow: 0 20px 60px rgba(0, 0, 0, 0.3);
            width: 90vw;
            max-width: 900px;
            height: 90vh;
            max-height: 800px;
            display: flex;
            flex-direction: column;
            overflow: hidden;
            animation: slideIn 0.6s cubic-bezier(0.68, -0.55, 0.265, 1.55);
        }

        @keyframes slideIn {
            from {
                opacity: 0;
                transform: translateY(-30px) scale(0.95);
            }
            to {
                opacity: 1;
                transform: translateY(0) scale(1);
            }
        }

        .header {
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            color: white;
            padding: 25px 30px;
            text-align: center;
            border-radius: 20px 20px 0 0;
        }

        .header h1 {
            font-size: 1.8rem;
            font-weight: 600;
            margin-bottom: 8px;
        }

        .progress-info {
            font-size: 0.95rem;
            opacity: 0.95;
            font-weight: 500;
        }

        .content {
            flex: 1;
            display: flex;
            flex-direction: column;
            padding: 30px;
            overflow-y: auto;
            overflow-x: hidden;
        }

        .content::-webkit-scrollbar {
            width: 8px;
        }

        .content::-webkit-scrollbar-track {
            background: #f1f1f1;
            border-radius: 10px;
        }

        .content::-webkit-scrollbar-thumb {
            background: #667eea;
            border-radius: 10px;
        }

        .question-section {
            margin-bottom: 25px;
        }

        .question-text {
            font-size: 1.25rem;
            color: #2d3748;
            font-weight: 600;
            line-height: 1.6;
            margin-bottom: 25px;
        }

        .choices {
            display: flex;
            flex-direction: column;
            gap: 12px;
        }

        .choice {
            background: #f7fafc;
            border: 2px solid #e2e8f0;
            border-radius: 12px;
            padding: 16px 20px;
            cursor: pointer;
            transition: all 0.3s ease;
            font-size: 1.05rem;
            color: #2d3748;
            position: relative;
            overflow: hidden;
        }

        .choice:hover:not(.disabled) {
            background: #edf2f7;
            border-color: #667eea;
            transform: translateX(5px);
        }

        .choice.selected-correct {
            background: #c6f6d5;
            border-color: #48bb78;
            animation: correctPulse 0.6s ease;
        }

        @keyframes correctPulse {
            0%, 100% { transform: scale(1); }
            50% { transform: scale(1.02); }
        }

        .choice.selected-incorrect {
            background: #fed7d7;
            border-color: #f56565;
            animation: incorrectShake 0.5s ease;
        }

        @keyframes incorrectShake {
            0%, 100% { transform: translateX(0); }
            25% { transform: translateX(-5px); }
            75% { transform: translateX(5px); }
        }

        .choice.faded {
            opacity: 0.5;
        }

        .choice.disabled {
            cursor: not-allowed;
        }

        .explanation-section {
            background: #edf2f7;
            border-radius: 12px;
            padding: 20px;
            margin-top: 20px;
            border-left: 4px solid #667eea;
            animation: fadeInUp 0.5s ease;
            max-height: 200px;
            overflow-y: auto;
            position: relative;
        }

        .reset-button {
            background: linear-gradient(135deg, #f093fb 0%, #f5576c 100%);
            color: white;
            border: none;
            padding: 10px 20px;
            border-radius: 8px;
            font-size: 0.9rem;
            font-weight: 600;
            cursor: pointer;
            margin-top: 15px;
            transition: all 0.3s ease;
            box-shadow: 0 4px 15px rgba(245, 87, 108, 0.3);
        }

        .reset-button:hover {
            transform: translateY(-2px);
            box-shadow: 0 6px 20px rgba(245, 87, 108, 0.5);
        }

        .explanation-section::-webkit-scrollbar {
            width: 6px;
        }

        .explanation-section::-webkit-scrollbar-thumb {
            background: #667eea;
            border-radius: 10px;
        }

        @keyframes fadeInUp {
            from {
                opacity: 0;
                transform: translateY(10px);
            }
            to {
                opacity: 1;
                transform: translateY(0);
            }
        }

        .explanation-section h3 {
            color: #667eea;
            font-size: 1.1rem;
            margin-bottom: 12px;
            font-weight: 600;
        }

        .explanation-section p {
            color: #4a5568;
            line-height: 1.6;
            font-size: 0.95rem;
            margin-bottom: 8px;
        }

        .navigation {
            display: flex;
            justify-content: space-between;
            padding: 20px 30px;
            border-top: 2px solid #e2e8f0;
            background: #f7fafc;
        }

        .nav-button {
            background: linear-gradient(135deg, #667eea 0%, #764ba2 100%);
            color: white;
            border: none;
            padding: 12px 28px;
            border-radius: 10px;
            font-size: 1rem;
            font-weight: 600;
            cursor: pointer;
            transition: all 0.3s ease;
            box-shadow: 0 4px 15px rgba(102, 126, 234, 0.4);
        }

        .nav-button:hover:not(:disabled) {
            transform: translateY(-2px);
            box-shadow: 0 6px 20px rgba(102, 126, 234, 0.6);
        }

        .nav-button:disabled {
            opacity: 0.5;
            cursor: not-allowed;
            box-shadow: none;
        }

        .level-badge {
            display: inline-block;
            padding: 4px 12px;
            border-radius: 20px;
            font-size: 0.85rem;
            font-weight: 600;
            margin-left: 10px;
        }

        .level-easy {
            background: #c6f6d5;
            color: #22543d;
        }

        .level-medium {
            background: #feebc8;
            color: #7c2d12;
        }

        .level-hard {
            background: #fed7d7;
            color: #742a2a;
        }

        @media (max-height: 700px) {
            .header {
                padding: 15px 20px;
            }
            
            .header h1 {
                font-size: 1.4rem;
            }
            
            .content {
                padding: 20px;
            }
            
            .question-text {
                font-size: 1.1rem;
                margin-bottom: 15px;
            }
            
            .choice {
                padding: 12px 16px;
                font-size: 0.95rem;
            }
            
            .explanation-section {
                padding: 15px;
                max-height: 150px;
            }
            
            .navigation {
                padding: 15px 20px;
            }
        }

        @media (max-width: 600px) {
            .container {
                width: 95vw;
                height: 95vh;
            }

            .header h1 {
                font-size: 1.4rem;
            }

            .question-text {
                font-size: 1.05rem;
            }

            .choice {
                font-size: 0.95rem;
            }

            .nav-button {
                padding: 10px 20px;
                font-size: 0.9rem;
            }
        }