Chang’e­2 did many experiments, I think are of much help for knowing space.A. whichB.

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(答案→)A. which
B. what
C. when
D. where
解析：考查定语从句。“嫦娥二号”做了许多试验，我认为这些试验对人们了解太空很有帮助。I think是插入语，可忽略，定语从句缺少主语，排除C、D；what不引导定语从句。故选A。