题目内容
According to the passage, the ocean generally has more salt in ______.
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Which of the following is NOT mentioned in the first as a reason for popular interest in birds？
The use of heat pumps has been held back largely by skepticism about advertisers' claims that heat pumps can provide as many as two 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(致冷剂) that cycles alternatively from its liquid phase to its vapor phase in a closed loop. The refrigerant, starting as a low-temperature, lowpressure vapor, enters a compressor 'driven by an electric motor. The refrigerant leaves the compressor as a hot, dense vapor and flows through a heat exchanger called the condenser, which transfers heat from the refrigerant to a body of air. Now the refrigerant, as a high-pressure, cooled liquid confronts a flow restriction which causes the pressure to drop. As the pres
sure 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 o{ 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 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 compressor at one time. The heating capacity is proportional to this mass flow rate: the less the mass of refrigerant being compressed, the less the thermal load it can transfer through the heat-pump cycle. The volume {low rate of refrigerant vapor through the single-speed rotary compressor used in heat pumps is approximately constant. But cold refrigerant vapor entering a compressor is at lower pressure than warmer vapor. Therefore, the mass of cold refrigerant—and thus the thermal energy it carries—is less than ii the refrigerant vapor were warmer before compression.
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 enough heat.
The primary purpose of the passage is to ______.
People can't survive without plants.
Part science and part natural history, ornithology(鸟类学) owes its tremendous popularity as a science and a scientific recreation to a simple circumstance that is sometimes forgotten when we weigh the eligibility(有被选资格，合适)of different kinds of animals for informal study. Birds are day animals where others, just as engaging in their way—mice, for instance—are creatures of the night. Apart from their visibility (something very much in their favor), the beauty of birds and their many appealing habits have won for them a huge fan following that has more than once tempted impatient and ill-informed laboratory biologists to dismiss ornithology as so much bird-watching—an absurdly unjust judgment that ignores the fact that ornithology has made a number of profoundly important contributions to general biology.
Professional biologists are not hard put to reel off(滔滔不绝地讲) a list of distinctive contributions that ornithology has made to general biology science. They will cite, for example, Ernst Mayr's investigations of evolution and the mechanisms of speciation (物种形成), or M. R. Irwin's work on the immunologic performance of species of Columbidae(pigeons and doves) and of hybrids(杂交种)between them. Then again, David Lack's studies on mortality in wild populations of birds are the most illuminating life tables of wild animals and animal demography (统计学) generally. Most important of all, perhaps is the contribution that ornithology has made to the study of animal behavior, beginning with Julian Huxley' s classic study of the courtship habits of the great crested great and followed by Niko Tinbergen's studies on the behavior. of herring gulls.
What is the main idea of the passage？
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