GRE-练习二十三及答案解析.doc

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1、GRE-练习二十三及答案解析(总分：100.00，做题时间：90 分钟)一、BTranslation/B(总题数：8，分数：100.00)1.UBefore 1965 many scientists pictured the circulation of the oceans water mass as consisting of large, slow-moving currents, such as the Gulf Stream./U UThat view/U, based on 100 years of observations made around the globe, Uprod

2、uced only a rough approximation of the true circulation./U But in the 1950s and the 1960s, Uresearchers began to employ newly developed techniques and equipment,/U including subsurface floats that move with ocean currents and emit identification signals, and ocean-current meters that record data for

3、 months at fixed locations in the ocean. UThese instruments disclosed an unexpected level of variability in the deep ocean./U Rather than being characterized by smooth, large-scale currents that change seasonally (if at all), the seas are dominated by what oceanographers call mesoscale fields: fluct

4、uating, energetic flows whose velocity can reach ten times the mean velocity of the major currents. Mesoscale phenomenathe oceanic analogue of weather systemsoften extend to distances of 100 kilometers and persist for 100 days (weather systems generally extend about 1,000 kilometers and last 3 to 5

5、days in any given area). UMore than 90 percent of the kinetic energy of the entire ocean may be accounted for by mesoscale variability rather than by large-scale currents. Mesoscale phenomena may, in fact, play a significant role in oceanic mixing, air-sea interactions, and occasionalbut far-reachin

6、gclimatic events such as E1 Nino,/U the atmospheric-oceanic disturbance in the equatorial Pacific that affects global weather patterns. UUnfortunately, it is not feasible to use conventional techniques to measure mesoscale fields./U To measure them properly, monitoring equipment would have to be lai

7、d out on a grid at intervals of at most 50 kilometers, with sensors at each grid point lowered deep in the ocean and kept there for many months. Because using these techniques would be prohibitively expensive and time-consuming, Uit was proposed in 1979 that tomography be adapted to measuring the ph

8、ysical properties of the ocean./U In medical tomography x-rays map the human bodys density variations (and hence internal organs); the information from the x-rays, transmitted through the body along many different paths, is recombined to form three-dimensional images of the bodys interior. It is pri

9、marily this multiplicative increase in data obtained from the multipath transmission of signals that accounts for oceanographers attraction to tomography: Uit allows the measurement of vast areas with relatively few instruments./U Researchers reasoned that low-frequency sound waves, because they are

10、 so well described mathematically and because even small perturbations in emitted sound waves can be detected, could be transmitted through the ocean over many different paths and that the properties of the oceans interiorits temperature, salinity, density, and speed of currentscould be deduced on t

11、he basis of how the ocean altered the signals. Their initial trials were highly successful, and ocean acoustic tomography was born.（分数：12.50）_2.UBracken fern has been spreading from its woodland strongholds for centuries,/U but the rate of encroachment into open countryside has lately increased alar

12、mingly throughout northern and western Britain. A tough competitor, bracken Ureduces the value of grazing land/U by crowding out other vegetation. The fern is itself poisonous to livestock, and also encourages proliferation of sheep ticks, which Unot only attack sheep but also transmit diseases/U. N

13、o less important to some people are brackens effects on threatened habitats and on the use of uplands for recreational purposes, even though many appreciate its beauty. UBiological controls may be the only economic solution./U One potentially cheap and self-sustaining method of halting the spread of

14、 bracken is to introduce natural enemies of the plant. Initially unrestrained by predators of their own, foreign predators are likely to be able to multiply rapidly and overwhelm intended targets. Because bracken occurs throughout the world, there is plenty of scope for this approach. Two candidates

15、, both moths from the Southern Hemisphere, are now being studied. UOf course, biological control agents can safely be released only if it can be verified that they feed solely on the target weed. The screening tests have so far been fraught with difficulties./U The first large shipment of moths succ

16、umbed to a disease. Growing enough bracken indoors is difficult, and the moths do not readily exploit cut stems. These are common problems with rearing insects for biological control. UOther problems can be foreseen. Policymakers need to consider many factors and opinions such as the cost of control

17、 compared to existing methods, and the impact of the clearance of bracken on the landscape, wildlife, and vegetation./U In fact, scientists already have much of the information needed to assess the impact of biological control of bracken, but it is spread among many individuals, organizations, and g

18、overnment bodies. The potential gains for the environment are likely to outweigh the losses because few plants, insects, mammals, and birds live associated only with bracken, and many would benefit from a return of other vegetation or from a more diverse mosaic of habitats. But legal consequences of

19、 attempts at biological control present a potential minefield. For example, many rural tenants still have the right of “estoyers“, the right to cut bracken as bedding for livestock and uses. What would happen if they were deprived of these rights? Once a biological control agent is released, it is d

20、ifficult to control its speed. What consideration is due landowners who do not want to control bracken? According to law, the release of the biological control agents must be authorized by the secretary of state for the environment. But Britain lacks the legal and administrative machinery to assembl

21、e evidence for and against release.（分数：12.50）_3.It is well known that biological changes at the molecular level have morphogenetic consequences, consequences affecting the formation and differentiation of tissues and organs. It is superfluous to point out that gene mutations and disturbances of the

22、biosynthetic processes in the embryo may result in abnormalities in the morphology (structure) of an organism. However, whereas much is known about causes and consequences at the molecular level, and in spite of an enormous accumulation of chemical and morphological data on embryos of various kinds,

23、 Uour understanding of how genes control morphogenesis is still far from complete. Perhaps one reason for this is that molecular biologists and morphologists speak different languages./U Whereas the former speak about messenger-RNA and conformational changes of protein molecules, the latter speak of

24、 ectoderms, hypoblasts, and neural crests. UOne solution to this predicament is to try to find some phenomena relevant to morphogenesis which both the molecular biologist and the morphologist can understand and discuss./U As morphogenesis must be basically the result of changes in behavior of the in

25、dividual cells, it seems logical to ask morphologists to describe the morphogenetic events observed in terms of changes in cellular contact, changes in the rate of proliferation of cells, or similar phenomena. Once this is done, it may be appropriate to ask questions about the molecular background f

26、or these changes. One may, for instance, ask whether variations in cell contact reflect alterations in the populations of molecules at the cell surface, or one may inquire about the molecular basis for the increased cell mobility involved in cell dispersion. Studies of this kind have been carried ou

27、t with cells released from tissues in various ways and then allowed to reveal their behavior after being spread out into a thin layer. In many cases, such cells show the ability to reaggregate, after which different cell types may sort themselves out into different layers and even take part in still

28、 more intricate morphogenetic events. But in most cases, the behavior of cells in the intact embryo is difficult to study because of the thickness and opacity of the cell masses. The sea urchin embryo, however, has the advantage that it is so transparent that each cell can be easily observed through

29、out development. Thus, Uby recording the development of a sea urchin embryo with time-lapse photography, the research scientist might discover previously unknown features of cellular behavior. Perhaps the study of the sea urchin in this manner can provide a medium by which the molecular biologist an

30、d the morphologist can begin communicating with each other more effectively about the way in which genes control morphogenesis./U（分数：12.50）_4.Although, recent years have seen substantial reductions in noxious pollutants from individual motor vehicles, the number of such vehicles has been steadily in

31、creasing consequently, Umore than 1O0 cities in the United States still have levels of carbon monoxide, particulate matter, and ozone/U (generated by photochemical reactions with hydrocarbons from vehicle exhaust) Uthat exceed legally established limits./U UThere is a growing realization that the on

32、ly effective way to achieve further reductions in vehicle emissions/Ushort of a massive shift away from the private automobileUis to replace conventional diesel fuel and gasoline with cleaner-burning fuels/U such as compressed natural gas, liquefied petroleum gas, ethanol, or methanol. UAll of these

33、 altematives are carbon-based fuels whose molecules are smaller and simpler than those of gasoline./U These molecules burn more cleanly than gasoline, in part because they have fewer, if any, carbon-carbon bonds, and the hydrocarbons they do emit are less likely to generate ozone. The combustion of

34、larger molecules, which have multiple carbon-carbon bonds, involves a more complex series of reactions. These reactions increase the probability of incomplete combustion and are more likely to release uncombusted and photochemically active hydrocarbon compounds into the atmosphere. UOn the other han

35、d, alternative fuels do have drawbacks. Compressed natural gas would require that vehicles have a set of heavy fuel tanks/Ua serious liability in terms of performance and fuel efficiencyUand liquefied petroleum gas faces fundamental limits on supply./U UEthanol and methanol, on the other hand, have

36、important advantages over other carbon-based alternative fuels:/U they have a higher energy content per volume and would require minimal changes in the existing network for distributing motor fuel. UEthanol is commonly used as a gasoline supplement, but it is currently about twice as expensive as me

37、thanol,/U the low cost of which is one of its attractive features. Methanols most attractive feature, however, is that it can reduce by about 90 percent the vehicle emissions that form ozone, the most serious urban air pollutant. Like any alternative fuel, methanol has its critics. Yet much of the c

38、riticism is based on the use of “gasoline clone“ vehicles that do not incorporate even the simplest design improvements that are made possible with the use of methanol. It is true, for example, that a given volume of methanol provides only about one-half of the energy that gasoline and diesel fuel d

39、o; other things being equal, the fuel tank would have to be somewhat larger and heavier. However, since methanol-fueled vehicles could be designed to be much more efficient than “gasoline clone“ vehicles fueled with methanol, they would need comparatively less fuel. UVehicles incorporating only the

40、simplest of the engine improvements that methanol makes feasible would still contribute to an immediate lessening of urban air pollution./U（分数：12.50）_5.A mysterious phenomenon is the ability of over-water migrants to travel on course. Birds, bees, and other species can keep track of time without any

41、 sensory cues from the outside world, and such “biological clocks“ clearly contribute to their “compass sense“. For example, they can use the position of the Sun or stars, along with the time of day, to find north. But compass sense alone cannot explain how birds navigate the ocean: after a flock tr

42、aveling east is blown far south by a storm, it will assume the proper northeasterly course to compensate. Perhaps, some scientists thought, migrants determine their geographic position on Earth by celestial navigation, almost as human navigators use stars and planets, but this would demand of the an

43、imals a fantastic map sense. Researchers now know that some species have a magnetic sense, which might allow migrants to determine their geographic location by detecting variations in the strength of the Earths magnetic field.（分数：12.50）_6.Flatfish, such as the flounder, are among the few vertebrates

44、 that lack approximate bilateral symmetry (symmetry in which structures to the left and right of the bodys midline are mirror images). UMost striking among the many asymmetries evident in an adult flatfish is eye placement:/U before maturity one eye migrates, so that in an adult flatfish both eyes a

45、re on the same side of the head. While in most species with asymmetries virtually all adults share the same asymmetry, members of the starry flounder species can be either left-eyed (both eyes on the left side of head) or fight-eyed. UIn the waters between the United States and Japan,/U the starry f

46、lounder populations vary from about 50 percent left-eyed off the United States West Coast, through about 70 percent left-eyed halfway between the United States and Japan, to nearly 100 percent left-eyed off the Japanese coast. UBiologists call this kind of gradual variation over a certain geographic

47、 range a “cline“ and interpret clines as strong indications that the variation is adaptive, a response to environmental differences./U For the starry flounder this interpretation implies that a geometric difference (between fish that are mirror images of one another) is adaptive, that left-eyedness in the Japanese starry flounder has been selected for, which provokes a perplexing questions: what is the selective advantage in having both eyes on one side rather than on the oth