单项选择题
A.direction of air flow within the cylinder
B.sequence of port opening
C.method of opening exhaust ports
D.volume of air admitted to the cylinder
单项选择题 Oxygen corrosion in auxiliary boilers is prevented by treating the boiler feed tank with either sodium （）
单项选择题 Operating a diesel engine for prolonged periods, with a closed freshwater cooling system, at temperatures lower than the normal design temperature can cause （）.
单项选择题 Improper maintenance of the fuel oil burners in an automatically fired auxiliary boiler, could result in （）