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

一个重200N的物体在水平拉力作用下匀速前进了25m，拉力撤销后，物体由于惯性又前进了10m。若拉力做的功为500J,则拉力的大小为       N。