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：考查only的倒装句：only+状语（介词短语，副词，状语从句）后面的主句要部分倒装，这句话only then是时间状语，后面的句子倒装，因为是同时发生，所以是过去时，did提到主语前面，句意：期末考试我不及格，只有那时我才知道学习多么的重要。选D。