【题目】I was on the highway when this car went past, followed by a police car. They _______ at least 150 kilometers an hour.
A.should have been driving
B.must have been driving
C.could have driven
D.would have driven

【答案】B
【解析】句意：我在高速路上的时候，这是他的车经过，后面跟着一辆警车。他们肯定至少150公里的时速行驶。must表推测，对过去正在进行的动作的推测。故选B。