Nobody knows for sure how dinosaurs died out 65 000 000 years ago. It because of sudden c

问题补充：

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

答案：

A. must have happened

B. would have happened

C. should have happened

(答案→)D. might have happened

解析：句意：没有人确切的知道56，000，000年前是怎么死的。有可能是因为它们生活环境的突然的改变。因为是Nobody knows，所以只是对过去的不确定的推测，用might have done过去可能做了…，must havedone过去一定做了，should have done过去本应该做...would have done过去本来会做…，选D。