主体信号是指（）某段线路或闭塞分区的信号。
参考答案：直接防护
The hat borrowed from the drama company was so dirty that it required ________ thoroughly.
A．washing
B．being washed
C．to wash
D．washed
AR46的加密板是安装在哪类单板板上：（）
A.RPU
B.ERPU
C.FIC
D.LPU
