[单选题]驾驶舱顶板上的CMC按钮开关的作用是( ) the role of CMC switches on the cockpit overhead panel is A.按下此按钮,CMC1工作 Press this button, CMC1 operates
B. 按下此按钮,CMC2不工作
Press this button, CMC2 not operates C.此按钮提供一部 CMC向另一部CMC的人工转换
This button provides a CMC to another CMC mannully conversion