单项选择题
Which of the following terms is used to identify the pressure of the liquid entering a pump?（）
A.Suction head B.Pump head C.Discharge head D.Total head
单项选择题 Ammonia when used as a refrigerant is valuable because of its high efficiency, it is however （）.
单项选择题 Which of the following is operated from the main engine room console on an automated vessel?（）
单项选择题 An un-loader is installed on an air compressor to （）.
