It was quite a serious accident, caused by careless driving.
A. which I think was
B. I think which was
C. which I think it was
D. I think which it was
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(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：选A。本题考查由which引导的非限制性定语从句，which替代先行词accident 在定语从句中作主语。I think 是插入语，放在which后面。