He made another wonderful discovery ,____ of great importanc

He made another wonderful discovery ,____ of great importance to science.（a）
A.which I think is B.which I think it is C.which I think it D.I think which is 我知道选第一个,但是划线的不是在宾语从句中作主语吗,既然作主语,就是主语从句.那么像which这样的词不就不能用吗,为什么呀
请看清楚我疑惑的地方在哪，不要乱答，什么非限制性定从，我懂