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
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(答案→)A. which I think is
B. which I think it is
C. which I think it
D. I think which is
解析：考查定语从句。句中I think为插入语，先行词为discovery，在定语从句中作主语，故选A。句意：他又有一个杰出的发现，对科学有很大的作用。