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.
A.positive displacement B.ejector C.centrifugal D.air compressor
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 （）.
单项选择题 A centrifugal pump produces the required pressure energy by means of a rotating （） provided with vanes.
