Only after spending several nights working on the math problem to wonder whether the probl

Only after spending several nights working on the math problem to wonder whether the problem was wrongly set.
A. had he begun
B. he began
C. did he begin
D. began he

A. had he begunB. he began(答案→)C. did he beginD. began he解析：only+状语位于句首要采用倒装，又因为叙述过去的事情所以用一般过去时态。句意：只有在花费数晚上的时间计算这道题之后他才怀疑是否问题出错了。

我明天再问问老师，叫他解释下这个问题