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

问题补充：

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

答案：

A. had he begun

B. he began

(答案→)C. did he begin

D. 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。