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) reciprocating
(2) rotary
(3) centrifugal

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

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单项选择题

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

单项选择题

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

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 tree