在列车中某辆车排风口不排风，产生自然缓解，则为（）故障。
A.三通阀
B.制动缸
C.机车
D.安全阀
参考答案：B
We ______in this school for about two years.[ ]
A. study .
B. studied
C. will study
D. have studied
推送挂车的距离与停留车辆不足十车时，应显示（）信号。
A.减速
B.连挂
C.五车距离
D.三车距离
