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

Only after spending several nights working on the math problem         to wonder whether the problem was wrongly set.

1. A.
   had he begun
2. B.
   he began
3. C.
   did he begin
4. D.
   began he

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

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

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