单项选择题
The donkey boiler may be given a bottom blow （）
A.to raise water level B.only while making steam C.to remove scum D.when fire are secured
单项选择题 When steam is being raised on a boiler, the water level will normally （）.
单项选择题 The furnace of an oil fired boiler should be （） before the operating burner is lit.
单项选择题 （） are mounted in pairs to protect the boiler against overpressure.
