单项选择题
A.central air condition system
B.high velocity system
C.low velocity system
D.single duct system
单项选择题 The volute of centrifugal pump also acts as a diffuser converting （）into pressure head.
单项选择题 A centrifugal impeller consists of a number of vanes curving （） from the direction of rotation.
单项选择题 When installed for bilge pumping or ballast duty, centrifugal pumps require a （） to remove air from the suction pipe.