33．Only after he had spent several nights working on the math problemto wonder whether the

问题补充：

33．Only after he had spent several nights working on the math problemto wonder whether the problem was wrongly set．

A. did he begin

B. he began

C. he did begin

D. began he

答案：

(答案→)A. did he begin

B. he began

C. he did begin

D. began he

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