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

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. 

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

• 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.