I’msorry,Bill.I_________thebooktoyoulastweek,butIwassobusythatIforgottodoso.A.musthavereturnedB.couldreturnC.shouldhavereturnedD.mightreturn

　　I’m sorry, Bill. I _________ the book to you last week, but I was so busy that I forgot to do so.

　　A.musthavereturned

　　B.couldreturn

　　C.shouldhavereturned

　　D.mightreturn