接车进路信号机设于接车进路（）始端，对（）指示运行条件。
参考答案：道岔区；到达列车
— I can’t get through to the general manager’s office.
—The line is busy. Someone ________ the telephone.
A．must have used
B．could use
C．could have used
D．must be using
平界面反射波的t0(X)曲线为一条抛物线.
