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

问题补充：

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

A. must have returned

B. could return

C. should have returned

D. might return

答案：

A. must have returned

B. could return

(答案→)C. should have returned

D. might return

解析：本题考查情态动词。句意：Bill， 很抱歉。上周我就该把书还你，可我太忙了。忘记了。should have done只用于谈论过去情况，主要有两个用法：一是用于推测过去已经发生的情况，二是用于指本该发生而实际上未发生的情况。