单项选择题
When steam is being raised on a boiler, the water level will normally （）.
A.drop as the boiler warms up B.rise as the boiler warms up C.remain unchanged until the boiler is hot enough D.rise and fall with the steam demand
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.
单项选择题 In a （） boiler the hot gases from the furnace pass through the tubes while the water is on the outside.
