单项选择题
A.the mechanical seal
B.packed stuffing boxes
C.either A or B
D.neither A nor B
单项选择题 As for a hydraulic vane pump, which of the following is in contact with the vanes during working: （） (1)the rotor (2)the pump shaft (3)the external surface of the housing
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.