单项选择题
The safety valves are mounted in pairs to （） the boiler against overpressure.
A.use B.release C.protect D.effect
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.
单项选择题 Impurities from the boiler water surface can be removed through the （）.
