单项选择题
What is the purpose of a relief valve?（）
A.to fill 2 tanks at the same time B.to use pump at a higher pressure C.to return liquid to suction side if discharge valve is closed D.All of the above are true
单项选择题 A （） pump is that where the volume of the pump chamber is alternately increased to draw the liquid in and then decreased to force the liquid out.
单项选择题 A reciprocating pump belongs to （） according to its working principle.
单项选择题 Shaft sealing in a centrifugal pump can be effected by （）.
