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 （）.

单项选择题 In the design of hydraulic piping and equipment consideration is given to minimize turbulence in the hydraulic fluid, as this will cause （）.