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單項(xiàng)選擇題

Which of the following modes of heat transfer does NOT require any physical contact between a warmer and a cooler substance?（）

A.Radiation
B.Conduction
C.Natural convection
D.All of the above

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單項(xiàng)選擇題

Which of the valves listed should be used either in the fully opened or the fully closed position?（）

A.A gate valve
B.A globe valve
C.Any check valve
D.Any needle valve

單項(xiàng)選擇題

If a refrigeration compressor is running continuously without lowering the temperature in the refrigerated space, the trouble may be （）.

A.a shortage of compressor oil
B.warm food in the refrigerator
C.excessive condenser cooling water
D.a shortage of refrigerant