单项选择题
What is the substance called which can readily be converted from a liquid into a vapor, and also from a vapor into liquid with a reasonably narrow range of pressure?（）
A.Refrigerator B.Refrigerant C.Cooling medium D.Brine
单项选择题 In a refrigeration plant the （） need defrosting.
单项选择题 Hot gas defrosting uses the hot gas from （） to defrost the evaporator.
单项选择题 The evaporating pressure in R22 vapor compression refrigeration plant should be （） atmospheric.
