问题
选择题
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
答案
D
题目分析：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。