單項選擇題A gate valve installed in a pipeline should be （）.
A.used in either the fully dosed or fully opened position
B.installed with the stem down
C.used for steam service only
D.used to throttle or regulate the flow of liquid
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1.單項選擇題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
2.單項選擇題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
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