Question

It was such a serious mistake ,______caused by carelessness!

A.which I think was B.which I think was
C.I think which was D.I think which it was
B，which I think it was

Answer 1

选A.我认为，这是个由粗心导致的严重的错误。
很显然，后面的句子是一个非限制性定语从句，所以肯定由which引导，以which开头，所以排除C和D。I think是个插入语。 which关系代词就代替了主语problem，所以不需要再有主语it，所以排除B。

Answer 2

句子存在主语 选CD句子就有两个主语了 不过你给的A和B一样 是不是有个选项是which i think it was 但我认为选A

Answer 3

选A
which （I think ）was caused by carelessness!

Answer 4

a
mistake 做先行词

Answer 5

B

追问

why

追答

I think 是插入语