单项选择题
In a （） the center screw is driven and the outer screws idle.
A.single screw pump B.double-screw pump C.triple screw pump D.centripetal pump
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.
单项选择题 The pressure head of a pump is given in the unit of （）.
