97免费在线观看视频,亚洲综合自拍网,黄色毛片免费观看,热久久综合网,免费看日产一区二区三区 狠狠操av,久久久涩涩涩,在线精品免费视频,人人插天天干,久久91精品国产91久久

單項選擇題

The purpose of the programmed purge cycle on an automatically fired auxiliary boiler is to()

A.cool the furnace to prevent pre-ignition
B.remove explosive vapors from the furnace
C.evaporate accumulated unburned fuel oil
D.provide sufficient air in the furnace to allow ignition of the fuel

微信掃碼免費搜題