单项选择题

Which of the following pumps will build up a bursting pressure or stall their motors if the discharge valves are closed?（）
(1) gear
(2) centrifugal
(3) screw

A.（1）and（2）
B.（1），（2）and（3）
C.（1）and（3）
D.（2）and（3）

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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

A.throttling the pressure line
B.changing the speed of the rotor
C.either A or B
D.None of the above is true

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

A.(1)only
B.(1)and(2)
C.(1),(2)and(3)
D.(2)and(3)