急求助~~GWD16-3

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

这题选D。。。请教前半句话是啥句式啊？要选D 前后2个句子没分号 也没连词不就错了么。。。求NN。。