单项选择题
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
单项选择题 Which of the following modes of heat transfer does NOT require any physical contact between a warmer and a cooler substance?（）
单项选择题 Which of the valves listed should be used either in the fully opened or the fully closed position?（）
单项选择题 If a refrigeration compressor is running continuously without lowering the temperature in the refrigerated space, the trouble may be （）.