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

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.