The astronaut did many experiments in the spaceship, much help for knowing space.
A. which we think it is
B. which we think are of
C. of which we think are
D. I think which is of
网友回答
A. which we think it is
(答案→)B. which we think are of
C. of which we think are
D. I think which is of
解析：句意：宇航员在宇宙飞船里做了许多实验，我们认为这些实验对理解太空知识很有帮助。转换句子结构，答案就会显露出来：…and we think they(the experiments) are of much help for knowing space.
be of much help=be very helpful。which的先行词为“the experiments”；we think变成了插入语。去掉这个插入语，答案更加明了。