He made another wonderful discovery, ________ 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

答案及解析： A

解析：

这是一个非限制性定语从句，make another wonderful discovery是动宾词组，关系代词which在定语从句中作主语，代表the discovery, I think为插入语．