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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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
A.throttling the pressure line
B.changing the speed of the piston
C.either A or B
D.None of the above is tree
A.throttling the pressure line
B.changing the speed of the rotor
C.either A or B
D.None of the above is tree
A.kinetic；pressure
B.kinetic；essential
C.pressure；kinetic
D.pressure；height
A.throttling the pressure line
B.changing the speed of the rotor
C.either A or B
D.None of the above is tree
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