單項選擇題R-12 is generally considered to be a safe, nontoxic, nonflammable, and non-explosive refrigerantIt can, however, become highly toxic when （）.

A.superheated outside the system
B.in contact with an open flame
C.heated to the boiling point
D.All of the above


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1.單項選擇題Which of the following statements is/are true regarding hydraulic pumps in general? （）

A.Variable volumes can be obtained with gear pumps only by variation of the pump drive speed
B.A radial piston pump houses sliding pistons in a stationary cylinder block through which passes a rotating pintle or ported shaft
C.The amount of fluid displaced per revolution of an axial piston rotary pump is dependent on the angle formed between the cylinder block and valve plate
D.All of the above

2.單項選擇題The function of the hydraulic telemotor transmitter used in an electro-hydraulic steering gear system is to （）.

A.transmit the rudder angle to the bridge indicator
B.prevent the control linkage from striking the stops when hard over
C.automatically purge all entrained air from the system
D.send hydraulic signals to the receiving unit

3.單項選擇題To obtain maximum efficiency, two stage air compressors are usually （）.

A.horizontally mounted
B.never fitted with intake filters
C.fitted with intercoolers
D.operated in an unloaded condition at all times

4.單項選擇題Which of the following statements is true concerning the overall efficiency of air compressors?（）

A.High pressure compressors are more efficient than low pressure compressors
B.Mechanical efficiency divided by compressor efficiency will equal the overall efficiency of the compressor unit
C.Two stage compressors are generally more efficient than single stage compressors
D.The volumetric efficiency is decreased in multistage compressors

5.單項選擇題The ability of an oil to separate cleanly from an oil and water mixture is referred to as its （）.

A.precipitation number
B.neutralization number
C.pour point
D.demulsibility