单项选择题
Generally the function of lubrication is to form of a film of lubricant between any two surfaces that have relative （）, so that surfaces dont get touch with each other.
A.gravity B.motion C.ratio D.density
单项选择题 Fuels of high quality with their earlier ignitions will cause higher （）pressure in cylinders of a diesel engine.
单项选择题 Fuel pump index is （）.
单项选择题 Fuel oil is thought to be one of the main factors （） the operation and maintenance of an engine.
