Mark the letter A, B,

Câu hỏi: Mark the letter A, B, C or D on your answer sheet to indicate the sentence that best combines each pair of sentences in the following questions.

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

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

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

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