单项选择题 The internal part of the boiler, if found （）, has to be chemically washed by yard.

单项选择题 Centrifugal pumps can be designed with a number of impellers （） to attain a high final delivery pressure.

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