单项选择题
A.Protects against suction valve float
B.Provides a positive pressure on the air inlet valves
C.Prevents lubricating oil contamination of the compressed air supply
D.Protects against the damaging effects of airborne solid particles
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 （）.
单项选择题 The viscosity of an oil is a measure of its （）.