You must have been driving at seventy miles an/per hour.

问题描述:

You must have been driving at seventy miles an/per hour.
你刚才一定是以每小时70英里的速度开车.
1.为什么要用ing

must have been doing
是must 表示对【过去正在发生的】事情的推测.
希望我的回答能够帮到你,