问题

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 happened
B:would have happened
C:should have happened
D:might have happened

正确答案是：

同类题型

The most exciting thing for him was____he finally succeeded in____seemed to be the most difficult exam to him.

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ou have a good sleep last night —Yes, never sleep .