问题补充：
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
答案：
(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。
