问题补充：
It was quite a serious mistake, caused by carelessness.
A. which I think was
B. I think which was
C. which I think it was
D. I think which it was
答案：
A. which I think was
B. I think which was
C. which I think it was
D. I think which it was
解析：which引导非限制性定语从句, I think是插入语
