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

问题补充：

I failed in the 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：句意：上学期期末考试我没考及格，只在那时我才意识到学习的重要性。Only修饰状语then位于句首时需使用部分倒装，本句的时态应使用一般过去式。