单项选择题
A.an diesel oil heater
B.a waste heat boiler
C.a fresh water generator
D.all the above
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.
单项选择题 The safety valves should be tested and adjusted when the boiler survey is （）