单项选择题
Which of the following pumps belongs to centrifugal pump?（）
A.Piston pump B.Screw pump C.Diffuser pump D.Water-ring pump
单项选择题 The pump discharge pressure will be considerably higher than the pressure at the （）.
单项选择题 What is the purpose of a relief valve?（）
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.
