单项选择题 In motor ship, （） is often used to recover some of the heat carried in the exhaust gases from the main engine.

单项选择题 Impurities from the boiler water surface can be removed through the （）.

单项选择题 You should frequently compare drum-mounted and remote-reading water level indicators（） between these have contributed to failures because of overheating through shortage of water, when a boiler was being oil-fired.