单项选择题
A.providing the impeller with balancing holes
B.the use of symmetrical impeller
C.neither A nor B
D.either A or B
单项选择题 The amount of fluid delivered by a gear pump can be regulated by （）.
单项选择题 Which of the following pumps are self-priming, and can deal with liquid from a level below the pump（） (1) ejector (2) screw (3) centrifugal
单项选择题 In （） pumps a closed discharge valve will cause bursting discharge pressure.