单项选择题

Which of the listed characteristics of lube oil are the most important to the engineer from an operational standpoint?（）

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

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单项选择题

Which of the following statements describes the function of an air compressor intake filter?（）

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

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Hydraulic cranes must be properly warmed-up before being operated because （）.

A.warm-up allows the hydraulic system to become charged with oil
B.warm-up allows the relief valves to be properly tested
C.hydraulic strainers operate only during the warm-up period
D.hydraulic fluid must be at the proper viscosity

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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 （）.

A.are exposed to a vacuum
B.come in contact with extremely hot surfaces
C.are reduced in temperature to just above the pour point
D.become extremely agitated or aerated