169.A square wave is produce when there is a flow of electrons for a set period that stops for a set amount of time and then repeats ( )

　当在设定的周期内存在电子流,该电子流在设定的时间内重复,然后停止时,方波就会产生。

　方波是产生的,当有一段时间的电子流,停止一段时间,然后重复。

　当在设定的周期内存在电子流,该电子流在设定的时间内停止,然后重复时,方波就会产生。

正确答案：C

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