单项选择题
（） provides the head or pressure to supply the domestic water where required.
A.The compressed air B.The compressing air C.The transfer pump D.The head tank
单项选择题 Under rotary pumps are the gear, （）, cam-and-piston and screw types.
单项选择题 The water level in the gauge glass when a boiler is operating is （） when the boiler is shut down.
单项选择题 Large pipes or （） are fitted between the steam and water drums to ensure good natural circulation of water.
