单项选择题
How often should the lubricating oil of a diesel engine be changed?（）
A.After every trip B.Every 4000 hours C.According to manufacturer’s instructions D.Every time they are shutdown
单项选择题 The main function of piston compression rings is to （）
单项选择题 Modern marine diesel engines using mechanical fuel injection, operate on a combustion cycle which is（）
单项选择题 The function of the piston compression rings used in a diesel engine is to （）
