掌握了gre阅读里的长难句，到了实战演习的时候了。gre阅读练习每日一篇帮助gre考生循序渐进地进行练习和总结。希望gre考生在进行gre阅读练习时，也按着考试时候的时间规定自己的练习，这样才能有效果。

　　The molecules of carbon dioxide in the Earth’s atmosphere affect the heat balance of the Earth by acting as a one-way screen. Although these molecules allow radiation at visible wavelengths, where most of the energy of sunlight is concentrated, to pass through, they absorb some of the longer-wavelength, infrared emissions radiated from the Earth’s surface, radiation that would otherwise be transmitted back into space. For the Earth to maintain a constant average temperature, such emissions from the planet must balance incoming solar radiation. If there were no carbon dioxide in the atmosphere, heat would escape from the Earth much more easily. The surface temperature would be so much lower that the oceans might be a solid mass (solid mass: 实体) of ice.

　　Today, however, the potential problem is too much carbon dioxide. The burning of fossil fuels and the clearing of forests have increased atmospheric carbon dioxide by about 15 percent in the last hundred years, and we continue to add carbon dioxide to the atmosphere. Could the increase in carbon dioxide cause a global rise in average temperature, and could such a rise have serious consequences for human society? Mathematical models that allow us to calculate the rise in temperature as a function of the increase indicate that the answer is probably yes.

　　Under present conditions a temperature of-18℃ can be observed at an altitude of 5 to 6 kilometers above the Earth. Below this altitude (called the radiating level), the temperature increases by about 6℃ per kilometer approaching the Earth’s surface, where the average temperature is about 15℃. An increase in the amount of carbon dioxide means that there are more molecules of carbon dioxide to absorb infrared radiation. As the capacity of the atmosphere to absorb infrared radiation increases, the radiating level and the temperature of the surface must rise.

　　One mathematical model predicts that doubling the atmospheric carbon dioxide would raise the global mean surface temperature by 2.5℃. This model assumes that the atmosphere’s relative humidity (relative humidity: n.相对湿度) remains constant and the temperature decreases with altitude at a rate of 6.5℃ per kilometer. The assumption of constant relative humidity is important, because water vapor in the atmosphere is another ficient absorber of radiation at infrared wavelengths. Because warm air can hold more moisture than cool air, the relative humidity will be constant only if the amount of water vapor in the atmosphere increases as the temperature rises. Therore, more infrared radiation would be absorbed and reradiated back to the Earth’s surface. The resultant warming at the surface could be expected to melt snow and ice, reducing the Earth’s rlectivity. More solar radiation would then be absorbed, leading to a further increase in temperature.

　　17. The primary purpose of the passage is to

　　(A) warn of the dangers of continued burning of fossil fuels

　　(B) discuss the significance of increasing the amount of carbon dioxide in the atmosphere

　　(C) explain how a constant temperature is maintained on the Earth’s surface

　　(D) describe the ways in which various atmospheric and climatic conditions contribute to the Earth’s weather

　　(E) demonstrate the usulness of mathematical models in predicting long-range climatic change

　　18. According to the passage, the greatest part of the solar energy that reaches the Earth is

　　(A) concentrated in the infrared spectrum

　　(B) concentrated at visible wavelengths

　　(C) absorbed by carbon dioxide molecules

　　(D) absorbed by atmospheric water vapor

　　(E) rlected back to space by snow and ice

　　19. According to the passage, atmospheric carbon dioxide performs all of the following functions EXCEPT:

　　(A) absorbing radiation at visible wavelengths

　　(B) absorbing infrared radiation

　　(C) absorbing outgoing radiation from the Earth

　　(D) helping to retain heat near the Earth’s surface

　　(E) helping to maintain a constant average temperature on the Earth’s surface

　　20. Which of the following best describes the author’s attitude toward the increasing amount of carbon dioxide in the atmosphere and its consequences?

　　(A) Incredulous

　　(B) Completely detached

　　(C) Interested but skeptical

　　(D) Angry yet resigned

　　(E) Objective yet concerned

　　21. It can be concluded from information contained in the passage that the average temperature at an altitude of 1 kilometer above the Earth is about

　　(A) 15℃

　　(B) 9℃

　　(C) 2.5℃

　　(D) -12℃

　　(E) -18℃

　　22. It can be inferred from the passage that the construction of the mathematical model mentioned in the passage involved the formulation of which of the following?

　　(A) An assumption that the amount of carbon dioxide added to the atmosphere would in reality steadily increase

　　(B) An assumption that human activities are the only agencies by which carbon dioxide is added to the atmosphere

　　(C) Assumptions about the social and political consequences of any curtailment of the use of fossil fuels

　　(D) Assumptions about the physical conditions that are likely to prevail during the period for which the model was made

　　(E) Assumptions about the differential behavior of carbon dioxide molecules at the various levels of temperature calculated in the model

　　23. According to the passage, which of the following is true of the last hundred years?

　　(A) Fossil fuels were burned for the first time.

　　(B) Greater amounts of land were cleared than at any time bore.

　　(C) The average temperature at the Earth’s surface has become 2℃ cooler.

　　(D) The amount of carbon dioxide in the atmosphere has increased measurably.

　　(E) The amount of farmland worldwide has doubled.

　　Some modern anthropologists hold that biological evolution has shaped not only human morphology but also human behavior. The role those anthropologists ascribe to evolution is not of dictating the details of human behavior but one of imposing constraints—ways of feeling, thinking, and acting that “come naturally” in archetypal situations in any culture. Our “frailties”—emotions and motives such as rage, fear, greed, gluttony, joy, lust, love—may be a very mixed assortment, but they share at least one immediate quality: we are, as we say, “in the grip” of them. And thus they give us our sense of constraints.

　　Unhappily, some of those frailties—our need for ever-increasing security among them—are presently maladaptive. Yet beneath the overlay of cultural detail, they, too, are said to be biological in direction, and therore as natural to us as are our appendixes. We would need to comprehend thoroughly their adaptive origins in order to understand how badly they guide us now. And we might then begin to resist their pressure.

　　24. The primary purpose of the passage is to present

　　(A) a position on the foundations of human behavior and on what those foundations imply

　　(B) a theory outlining the parallel development of human morphology and of human behavior

　　(C) a diagnostic test for separating biologically determined behavior patterns from culture-specific detail

　　(D) a practical method for resisting the pressures of biologically determined drives

　　(E) an overview of those human emotions and motives that impose constraints on human behavior

　　25. The author implies that control to any extent over the “frailties” that constrain our behavior is thought to presuppose

　　(A) that those frailties are recognized as currently benicial and adaptive

　　(B) that there is little or no overlay of cultural detail that masks their true nature

　　(C) that there are cultures in which those frailties do not “come naturally” and from which such control can be learned

　　(D) a full understanding of why those frailties evolved and of how they function now

　　(E) a thorough grasp of the principle that cultural detail in human behavior can differ arbitrarily from society to society

　　26. Which of the following most probably provides an appropriate analogy from human morphology for the “details” versus “constraints” distinction made in the passage in relation to (in relation to: adv.关于, 涉及, 与…相比) human behavior?

　　(A) The ability of most people to see all the colors of the visible spectrum as against most people’s inability to name any but the primary colors

　　(B) The ability of even the least fortunate people to show compassion as against people’s inability to mask their feelings completely

　　(C) The ability of some people to dive to great depths as against most people’s inability to swim long distances

　　(D) The psychological profile of those people who are able to delay gratification as against people’s inability to control their lives completely

　　(E) The greater lung capacity of mountain peoples that helps them live in oxygen-poor air as against people’s inability to fly without special apparatus

　　27. It can be inferred that in his discussion of maladaptive frailties the author assumes that

　　(A) evolution does not favor the emergence of adaptive characteristics over the emergence of maladaptive ones

　　(B) any structure or behavior not positively adaptive is regarded as transitory in evolutionary theory

　　(C) maladaptive characteristics, once fixed, make the emergence of other maladaptive characteristics more likely

　　(D) the designation of a characteristic as being maladaptive must always remain highly tentative

　　(E) changes in the total human environment can outpace evolutionary change

答案：17-27：BBAEBDDADEE

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