单项选择题
A.(1) only
B.(2) only
C.(1) and (2)
D.(1),(2)and(3)
单项选择题 As for a hydraulic single action vane pump, the inner wall of the stator is usually of （） shape.
单项选择题 Throttling in the （） line of a centrifugal pump involves the risk of the pump cavitation.
单项选择题 A positive displacement pump is that where the volume of the pump chamber is alternately increased to （） and then decreased to （）.