单项选择题
A.Ash content, carbon residue, and gravity
B.Viscosity, acidity, and demulsibility
C.Pour point, flash, point, and precipitation number
D.Auto-ignition point, viscosity index, and film strength
单项选择题 Which of the following statements describes the function of an air compressor intake filter?（）
单项选择题 Hydraulic cranes must be properly warmed-up before being operated because （）.
单项选择题 Although lube oils used in the main lubricating service systems should have a relatively high flash point to avoid ignition, they can create smoke and fire hazards when they （）.