A brief introduction is made of the design,operating principle, use and maintenance of the signalling device, as well as the significance of using such a device at coal face for safety production.是什么意思_A brief introduction is made of the design,operating principle, use and maintenance of the signalling device, as well as the significance of using such a device at coal face for safety production.在線(xiàn)翻譯_英語(yǔ)_讀音_用法_例句