GRE Reading Comprehension: ETS-GRE阅读ETS - OQ40N279YOSJ43FV7

According to the passage, the role of the flow restriction in a heat pump is to A. measure accurately the flow rate of the refrigerant mass at that point B. compress and heat the refrigerant vapor C. bring about the evaporation and cooling of refrigerant D. exchange heat between the refrigerant and the air at that point E. reverse the direction of refrigerant flow when needed