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。
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