新prep1-239（GWD-12-Q3）这是什么结构？

239. (28502-!-item-!-188;#058&003082) (GWD-12-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

答案是D/

这是什么结构，我为什么看不明白？

好象是两个并列，但是也没有连词，还有instead为什么可以不要？我刚开始选的A。

是跟 if 条件状语从句一样吗？if A happen, B will happen.

if it were not for
  the fusion-powered heat and radiation that rush from its core, a star would collapse under its own weight.

it refers to a star（主语）, A的主语是it's own weight所以错。

那么for是什么意义呢？