单项选择题
A.filled with water
B.purged
C.cleared of air
D.None of the above is true
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.
单项选择题 In a （） boiler the water flows through the inside of the tubes while the hot gases pass around the outside.