Only after I had reached the air

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
【题目解析】：【答案精解】C。句意：到机场后，我才发现忘记带飞机票了。本题考查倒装结构。当only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过去时．因此C项正确。