He made another wonderful discovery, ___A___ of great importance to science.
A. which I think is B. which I think it is
C. which I think it D. I think which is
这里是带有插入语的定语从句。I think是插入语,去掉插入语句子结构也不受影响。which引导非限制性定语从句修饰主句内容。再举个例子:
She could go to jail, which I guess is also my fault.