单项选择题
A screw pump belongs to （） according to its working principle.
A.reciprocating pumps B.positive displacement pumps C.centrifugal pumps D.centripetal pumps
单项选择题 Through a （） pump, the liquid is transferred from the suction to the discharge by the mechanical variation of the volume of a chamber or chambers.
单项选择题 In a （） the center screw is driven and the outer screws idle.
单项选择题 The discharge flow of a reciprocating pump is （）.
