I didn't pay attention to the teacher. I failed to understand the lesson.

40/40

I would have understood the lesson if I had paid attention to the teacher.

Although I paid attention to the teacher, I failed to understand the lesson.

I would have understood the lesson if I had failed to pay attention to the teacher.

Unless I failed to understand the lesson, I would pay attention to the teacher.

Giải thích

Đáp án đúng: A

Giải thích: Cấu trúc câu điều kiện loại 3: If S + had V3, S + would have V3

Dịch: Tôi sẽ hiểu bài nếu tôi chú ý đến giáo viên