He has made another wonderful discovery, ____ of great importanc

He has made another wonderful discovery, ____ of great importance to science.
A、which I think it is
B、of which I think it is
C、I think which is
D、which I think is
【正确答案】：D
【题目解析】：首先去掉插入语I think，使选项明朗化；因前后都是两个无连词的句子，可判断后句为非限制性定语从句。