单项选择题
The safety valve should be （） in the presence of C/E and Surveyor.
A.tightened up B.sealed up C.opened up D.unscrewed
单项选择题 The safety valves are mounted in pairs to （） the boiler against overpressure.
单项选择题 The defective place in the furnace should be （） with fire bricks.
单项选择题 The exhaust boilers are used to recover some of （） carried in the exhaust gas from the main engine.
