GMAT考试阅读资料(五)d

Passage 26
　　In choosing a method for determining climatic condi-tions that existed in the past， paleoclimatologists invokefour principal criteria¹. First， the material——rocks， lakes，vegetation， etc-on which the method relies must be（5） widespread enough to provide plenty of information，since analysis of material that is rarely encountered willnot permit correlation² with other regions or with otherperiods of geological history. Second， in the process offormation， the material must have received an environ-（10） mental signal that reflects a change in climate and thatcan be deciphered by modern physical or chemicalmeans. Third， at least some of the material must haveretained the signal unaffected by subsequent changes inthe environment. Fourth， it must be possible to deter-（15） mine the time at which the inferred climatic conditionsheld. This last criterion is more easily met in datingmarine sediments⁴， because dating of only a smallnumber of layers in a marine³ sequence allows the age ofother layers to be estimated fairly reliably by extrapola-（20） tion and interpolation. By contrast， because sedimenta-tion is much less continuous in continental⁵ regions， esti-mating the age of a continental bed from the knownages of beds above and below is more risky⁶.
　　One very old method used in the investigation⁷ of past（25） climatic conditions involves the measurement of waterlevels in ancient lakes. In temperate⁸ regions， there areenough lakes for correlations⁹ between them to give us areliable picture. In arid¹⁰ and semiarid¹¹ regions， on theother hand， the small number of lakes and the great（30） distances between them reduce the possibilities for corre-lation. Moreover， since lake levels are controlled by ratesof evaporation¹² as well as by precipitation， the interpreta-tion of such levels is ambiguous. For instance， the factthat lake levels in the semiarid southwestern United（35） States appear to have been higher during the last ice agethan they are now was at one time attributed toincreased precipitation. On the basis of snow-line eleva-tions， however， it has been concluded that the climatethen was not necessarily wetter than it is now， but rather（40） that both summers and winters were cooler， resulting in　 reduced evaporation.
　　Another problematic method is to reconstruct formerclimates on the basis of pollen¹³ profiles. The type of vege-tation in a specific region is determined¹⁴ by identifying（45） and counting the various pollen grains found there.Although the relationship between vegetation andclimate is not as direct as the relationship betweenclimate and lake levels， the method often works well inthe temperate zones. In arid and semiarid regions in（50） which there is not much vegetation， however， smallchanges in one or a few plant types can change thepicture dramatically， making accurate correlations　 between neighboring areas difficult to obtain.

　　1. Which of the following statements about the　difference between marine and continental　sedimentation¹⁵ is supported by information in the　passage？
　　　（A） Data provided by dating marine sedimentation is more consistent with researchers‘ findings in other disciplines than is data provided by dating continental sedimentation.
　　　（B） It is easier to estimate the age of a layer in a sequence of continental sedimentation than it is to estimate the age of a layer in a sequence of marine sedimentation.
　　　（C） Marine sedimentation is much less widespread than continental sedimentation.
　　　（D） Researchers are more often forced to rely on extrapolation when dating a layer of marine sedimentation than when dating a layer of continental sedimentation.
　　　（E） Marine sedimentation is much more continuous than is continental sedimentation.

　　2. Which of the following statements best describes the organization of the passage as a whole？
　　　（A） The author describes a method for determining past climatic conditions and then offers specific examples of situations in which it has been used.
　　　（B） The author discusses the method of dating marine and continental sequences and then explains how dating is more difficult with lake levels than with pollen profiles.
　　　（C） The author describes the common requirements of methods for determining past climatic conditions　and then discusses examples of such methods.
　　　（D） The author describes various ways of choosing a material for determining past climatic conditions and then discusses how two such methods have yielded contradictory¹⁶ data.
　　　（E） The author describes how methods for determining past climatic conditions were first developed and then describes two of the earliest known methods.

　　3. It can be inferred from the passage that　paleoclimatologists have concluded which of the　following on the basis of their study of snow-line　elevations¹⁷ in the southwestern United States？
　　　（A） There is usually more precipitation during an ice age because of increased amounts of evaporation.
　　　（B） There was less precipitation during the last ice age than there is today.
　　　（C） Lake levels in the semiarid southwestern United States were lower during the last ice age than they are today.
　　　（D） During the last ice age， cooler weather led to lower lake levels than paleoclimatologists had previously¹⁸ assumed.
　　　（E） The high lake levels during the last ice age may have been a result of less evaporation rather than more precipitation.

　　4. Which of the following would be the most likely topic for a paragraph that logically continues the passage？
　　　（A） The kinds of plants normally found in arid regions
　　　（B） The effect of variation in lake levels on pollen distribution
　　　（C） The material best suited to preserving signals of climatic changes
　　　（D） Other criteria invoked¹⁹ by paleoclimatologists when choosing a method to determine past climatic conditions
　　　（E） A third method for investigating past climatic conditions

　　5. The author discusses lake levels in the southwestern　United States in order to
　　　（A） illustrate²⁰ the mechanics of the relationship between lake level， evaporation， and precipitation
　　　（B） provide an example of the uncertainty²¹ involved in interpreting lake levels
　　　（C） prove that there are not enough ancient lakes with which to make accurate correlations
　　　（D） explain the effects of increased rates of evaporation on levels of precipitation
　　　（E） suggest that snow-line elevations are invariably more accurate than lake levels in determining rates of precipitation at various points in the past

　　6. It can be inferred from the passage that an　environmental signal found in geological material　would not be useful to paleoclimatologists if it
　　　（A） had to be interpreted by modern chemical means
　　　（B） reflected a change in climate rather than a long-term climatic condition
　　　（C） was incorporated into a material as the material was forming
　　　（D） also reflected subsequent environmental changes
　　　（E） was contained in a continental rather than a marine sequence

　　7. According to the passage， the material used to determine　past climatic conditions must be widespread for which of the following reasons？
　　　Ⅰ.Paleoclimatologists need to make comparisons between periods of geological history.
　　　Ⅱ. Paleoclimatologists need to compare materials that have supported a wide variety of vegetation.
　　　Ⅲ. Paleoclimatologists need to make comparisons with data collected in other regions.　
　　　（A） Ⅰ only
　　　（B） Ⅱ only
　　　（C） Ⅰ and Ⅱ only
　　　（D） Ⅰ and Ⅲ only
　　　（E） Ⅱ and Ⅲ only

　　8. Which of the following can be inferred from the passage　about the study of past climates in arid and semiarid　regions？
　　　（A） It is sometimes more difficult to determine pastclimatic conditions in arid and semiarid regions thanin temperate regions.
　　　（B） Although in the past more research has been done on temperate regions， paleoclimatologists have recently turned their attention to arid and semiarid regions.
　　　（C） Although more information about past climates can be gathered in arid and semiarid than in temperate regions， dating this information is more difficult.
　　　（D） It is difficult to study the climatic history of arid and semiarid regions because their climates have tended to vary more than those of temperate regions.
　　　（E） The study of past climates in arid and semiarid regions has been neglected because temperate regions support a greater variety of plant and animal life.