Only after he had spent several nights working on the math problem to wonder whether the

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

A. that he began (答案→)B. did he begin C. then he beganD. so that he began解析：only +状语位于句首要用部分到装。 句意：只有花费了数晚计算这道数学题之后他才开始怀疑是否问题出错了。

这下我知道了