It was a serious mistake, caused by carelessness.
A. which I think was
B. which I think it was
C. I think which was
D. that I think it was
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(答案→)A. which I think was
B. which I think it was
C. I think which was
D. that I think it was
解析：考查定语从句。句中I think为插入语，先行词为a serious mistake，在从句中作主语。句意：它是一个严重的错误，我认为是有粗心引起的。