Only when I checked the paper once more there was a mistake.A. I realizedB. did I realize

问题补充：

Only when I checked the paper once more there was a mistake.

A. I realized

B. did I realize

C. I had realized

D. had I realized

答案：

A. I realized

(答案→)B. did I realize

C. I had realized

D. had I realized

解析：句意：只有我再次检查试卷时才意识到有一个错误出现了。这是倒装句，Only+状语从句放在句首时，主句用部分倒装。检查时发现的错误，检查和意识到几乎同时发生所以用过去式，故选B。