单项选择题
A.discharge is continuous
B.it has no internal valves
C.upon accidental closing of discharge valve, excessive pressure will not build up
D.all of the above are true
单项选择题 A pump is said to have "negative suction head" when the pump is located （）.
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) vane (3) gear
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