I failed in the final examination last term and only then the importance of studies.A.

问题补充：

I failed in the final examination last term and only then the importance of studies.

A. I realized

B. I had realized

C. had I realized

D. did I realize

答案：

A. I realized

B. I had realized

C. had I realized

(答案→)D. did I realize

解析：此题考查倒装结构。only+副词、介词短语、状语从句并和这些成分一起提前时，句子要用部分倒装；而且我的醒悟不会在考试以前，所以不能选C项。