A.should have been doing C.could have done答案 B

B.must have been doing

D.would have done

解析 must have done用于肯定句中，表示对过去事情的推测，意为“想必是，一定是”。否定句、疑问句中对过去情况的推测，要用can/could have done的形式。 18.(05山东34)

He paid for a seat，however，he A.could 答案 A

解析 paid暗示时间上指的是过去，could have done用于肯定句中表示本可以做而未做的事情；肯定句中不用need have done; must have done与语境不吻合。 19.(05江西22)

—Tom is never late for work.Why is he absent today? —Something

to him.

B.should have happenedD.must have happened

B.would

have entered free. C.must

D.need

A.must happen

C.could have happened 答案 D

解析 本题考查情态动词的用法。must have done的意思是“一定发生了某事”。由对话所提供的情景可知：汤姆从不迟到，所以他“一定有事”。因此D项正确。 20.(05安徽26) Helen A.shall 答案 C

解析 此题考查情态动词的用法。may表示一种可能性。如果没有具体的语境的话，B、D两项也可以选，但是意思就不同了。 21.(05江苏31)

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go on the trip with us，but she isn’

t quite sure yet.

B.must

C.may

D.can