Only when he had done it___ that he had made a mistake.

Only when he had done it___ that he had made a mistake.
A、had he come to understand
B、he came to understand
C、he had come to understand
D、did he come to understand
【正确答案】：D
【题目解析】：句意：只有当他做了那件事，他才明白他犯了一个错误。