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
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C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。