单项选择题
Gear pumps are （） because they discharge a near constant amount of liquid regardless of pressure.
A.negative displacement type B.turbine type C.positive displacement type D.mixed-flow type
单项选择题 Keep compartment （） as the gas given off can be dangerous.
单项选择题 More than one stage of evaporation can take place by admitting the sea water into chambers with progressively （） pressure in the flash evaporator.
单项选择题 In the process of （）, liquid changes into gas.
