When he called to tell me that he was studying, I didn't believe him because I could hear the noise of a party in the background.  

A I didn't believe that he could study properly with the noise of a party in the background and I told him that when he telephoned.

B Although he rang me to tell me that he was studying, I couldn't hear what he was saying properly because of my unbelievably noisy party.

C Because of the sounds of a party I heard in the background when he phoned, I didn't believe his claim that he was studying.

D Though he was studying when he called, I thought he was lying because in the background there were party-like sounds.