虚拟语气GWD-12-3

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.

1. instead its own weight would cause a star to collapse
           
2. instead a star would have collapsed under its own weight
           
3. a star would have to be collapsing under its own weight
           
4. a star would collapse under its own weight
           
5. its own weight would have caused a star’s collapse

答案D     想问问B的问题在哪? would have done意思是本来可以怎么怎么样,记得以前高中时候虚拟语气中有   这样的用法,NN解答一下