单项选择题
A centrifugal pump produces the required pressure energy by means of a rotating （） provided with vanes.
A.propeller B.impeller C.circumference D.screw
单项选择题 The pressure head of a pump is given in the unit of （）.
单项选择题 A gear pump belongs to （） according to its working principle.
单项选择题 Gear pumps are of positive displacement type pumps because they discharge a near constant amount of liquid （） pressure.
