单项选择题
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
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 （）.
单项选择题 The pour point of lubricating oils is affected the most by which of the following?（）