问题补充：
He made another wonderful discovery, of great importance to science.
A. which I think
B. which I think it
C. which I think is
D. I think which is
答案：
A. which I think
B. which I think it
(答案→)C. which I think is
D. I think which is
解析：考查定语从句：句意：他又有了一个重大的发现，这我认为对科学是非常重要的。I think 是插入语，可以忽略，剩余的就是一个很明显的which指代前边整个句子（事实）引导的一个非限定性定语从句。Which在定语从句中做主语，it是多余的，定语从句中系表结构，is不能缺，还有插入语应该在which后面，选C。
