问题
选择题
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 doing
B．must have been doing
C．could have done
D．would have done
答案
B
此题考查虚拟语气。根据句意，我对这辆车和紧随其后的警车的速度不能确定，只能推测。“must have been doing” 表肯定，推测车速至少在每小时150千米。易错点:C项意为“本来能……(暗示实际上没能或没做)”。