小强的爸爸开汽车到距离xkm外的单位去上班，在正常情况下经过yh可以到达．但是有一天由于汽车需要维修晚出发zh，小强的爸爸每小时应该多走多少km，才能按时到达单位？

Not until the train pulled into the station          that his suitcase was gone.

A．did he find

B．had he found

C．he had found

D．he found