Only after you explained it to me it was not my teacher’s fault.
A. I realized
B. had I realized
C. realized I
D. did I realize
网友回答
A. I realized
B. had I realized
C. realized I
(答案→)D. did I realize
解析：考查倒装结构。Only放在句首，后接状语从句，主句的主谓要倒装。根据句意用一般过去时，选D。