单项选择题 A positive displacement pump is that where the volume of the pump chamber is alternately increased to （） and then decreased to （）.

单项选择题 The amount of fluid delivered by a helical 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) reciprocating (2) rotary (3) centrifugal