Nobody knows for sure how dinosaurs died out 65,000,000 years ago. It _______ because of sudden changes of their living conditions.

A．must have happenedB．would have happened
C．should have happenedD．might have happened

D

解析試題分析：句意：沒有人確切的知道56，000，000年前是怎么死的。有可能是因為它們生活環(huán)境的突然的改變。因為是Nobody knows，所以只是對過去的不確定的推測，用might have done過去可能做了…，must havedone過去一定做了，should have done過去本應該做...would have done過去本來會做…，選D。
考點：考查情態(tài)動詞的完成式

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