单项选择题
If a refrigeration compressor will not start, the reason may be （）.
A.the pressure regulating valve is not closing
B.an excessive lack of refrigerant in the system
C.badly leaking discharge valves
D.worn piston rings
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Which of the following statements describes the relationship between flash point and ignition temperature?（）
A.Both are higher than normal burning temperatures
B.The flash point is always higher
C.The ignition temperature is always higher
D.They are not necessarily related -
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Flexible hose under pressure in a hydraulic system will （）.
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B.expand in length and in diameter
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An air compressor can be unloaded at start-up by （）.
A.holding the discharge valve open
B.relieving the intercooler pressure to the atmosphere
C.using an enlarged, permanently opened clearance expansion space
D.a pre-charged accumulator
