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.
A.water-tube B.fire-tube C.exhaust gas D.none of the above
单项选择题 The tube in a boiler should be blinded if found （）.
单项选择题 The internal part of boiler should be welded by yard if found （）.
单项选择题 The internal part of the boiler, if found （）, has to be chemically washed by yard.
