单项选择题 Under normal conditions, the refrigerant enters the compressor in an operating refrigeration system as a （）.

单项选择题 Some vessels are equipped with a water lubricated stern robeWhen at sea, operating under normal conditions, the water service valve from the ships saltwater system to the bearing should be （）.

单项选择题 Sub-cooling is a method of reducing the temperature of the liquid refrigerant below its （）.