【单选题】-The use of heat pumps has’ been...

The use of heat pumps has’ been held back largely by skepticism about advertisers’ claims that heat pumps can provide as many as units of thermal energy for each unit of electrical energy used, thus apparently contradicting the principle of energy conservation. Heat pumps circulate a fluid refrigerant that cycles alternatively from its liquid phase to its vapor phase in a closed loop. The refrigerant, starting as a low-temperature, low-pssure vapor, enters compssor driven by an electric motor. The refrigerant leaves the compssor as a hot, dense vapor and flows through a heat exchanger called the condenser, which transfers heat from the refrigerant to a body or air. Now the refrigerant, as a high-pssure, cooled liquid, confronts a flow restriction which causes the pssure to drop.As the pssure falls, the refrigerant expands and partially vaporizes, becoming chilleD、It then passes through a second heat exchanger, the evaporator, which transfers heat from the air to the refrigerant, reducing the temperature of this second body of air. Of the two heat exchangers, one is located inside, and the other one outside the house, so each is in contact with a different body of air: room air and outside air, respectively.
　　The flow direction of refrigerant through a heat pump is controlled by valves. When the refrigerant flow is reversed, the heat exchangers switch function. This flow-reversal capability allows heat pumps--either to heat or cool room air.
　　Now, if under certain conditions a heat pump puts out more thermal energy than it consumes in electrical energy, has the law of energy conservation been challenged No, not even remotely: the additional input of thermal energy into the circulating refrigerant via the evaporator accounts for the difference in the energy equation.
　　Unfortunately, there is one real problem. The heating capacity of a heat pump decreases as the outdoor temperature falls. The drop in capacity is caused by the lessening amount of refrigerant mass moved through the compssor at one time. The heating capacity is proportional to this mass flow rate: the less the mass of refrigerant being compssed, the less the thermal load it can transfer through the heat-pump cycle. The volume flow rate of refrigerant vapor through the single-speed rotary compssor used in heat pumps is approximately constant.,But cold refrigerant vapor entering a compssor is at lower pssure than warmer vapor. Therefore, the mass of cold refrigerant--and thus the thermal energy it carries--is less than if the refrigerant vapor were warmer before compssion.
　　Here, then, lies a genuine drawback of heat pumps: in extremely cold climates--where the most heat is needed--heat pumps are least able to supply en6ugh heat.
　　The primary purpose, of the passage is to______
　　A、explain the differences in the working of a heat pump when the outdoor temperature changes
　　B、contrast the heating and the cooling modes of heat pumps
　　C、describe heat pumps, their use, and factors affecting their use