單項選擇題

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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單項選擇題

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

單項選擇題

The viscosity of an oil is a measure of its （）.

A.weight
B.internal friction
C.demulsibility