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 understand his explanation...

A. I didn’t understand his explanation because I asked him to repeat what he had said

B. I didn’t understand his explanation, and I asked him to repeat what he had said. 

C. I didn’t understand his explanation, for I asked him to repeat what he had said

D. I didn’t understand his explanation, so I asked him to repeat what he had said