移频自动闭塞，闭塞分区被占用或轨道电路失效时，防护该闭塞分区的通过信号机应自动关闭。
参考答案：对
Cars play an important role in modern life, but they ______ also cause many problems like air pollution.
A．should
B．can
C．must
D．will
对上料机的要求？
