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 （）.

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