单选题 Only after I had reached the airport ______ that I had forgotten to take my ticket with me.
A. I realized B. have I realized
C. did I realize D. I had realized

【正确答案】 C
【答案解析】[考点] 倒装结构
[解析] only+状语置于句首时，主句使用部分倒装结构，即将谓语部分的助动词置于主语之前。由从句中的过去完成时推知主句中应是一般过去时，故选C。