自己动手制作一套联络信号装置，既可解决前后呼应问题，又利于文明操作。
To do a set of connection signal apparatus by self can solve the question echoing each other in front and behind, and benefit civilized operation.
自己动手制作一套联络信号装置，既可解决前后呼应问题，又利于文明操作。
To do a set of connection signal apparatus by self can solve the question echoing each other in front and behind, and benefit civilized operation.
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