单项选择题
（） are mounted in pairs to protect the boiler against overpressure.
A.Safety valves B.Water tubes C.Steam stop valves D.Blow down valves
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.
单项选择题 The tube in a boiler should be blinded if found （）.
