GWD16-Q3求教

Q3:
Were it not for the fusion-powered heat and radiation that rush from its core, instead its own weight would cause a star to collapse.

A.    instead its own weight would cause a star to collapse
B.    instead a star would have collapsed under its own weight
C.    a star would have to be collapsing under its own weight
D.    a star would collapse under its own weight
E.    its own weight would have caused a star’s collapse
  Answer:  d
为什么答案是D呢？有人能帮忙解答吗？
还有想问下，有没有GWD的题目的解析一类的资料呢？我在CD上找了半天也没找到。。。烦请有的牛人发个链接或传个文件呗~谢谢。。。