继电器控制电路工作时,电路中硬件都处于受控状态,PLC各软继电器都处于周期循环扫描状态,各个软继电器的线圈和它的触点动作并不同时发生。 ( )
A campus emergency ______ occur at any time of the day or night, weekend, or holiday, with little or no warning.
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