问题补充:
He has made another wonderful discovery, of great importance to science.
A. who I think it
B. which I think it is
C. that I think is
D. which I think is
答案:
A. who I think it
B. which I think it is
C. that I think is
(答案→)D. which I think is
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。