【题目】He made another wonderful discovery，________of great importance.
A.which I think is
B.I think which is
C.which I think it is
D.which I think it

【答案】A
【解析】考查定语从句和插入语的用法。此题后句为which引导的定语从句，which在从句中作主语，I think是插入语，不影响句子结构，所以答案选A。