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  3. In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal cri

In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal cri

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问题     In choosing a method for determining climatic conditions that existed in the past, paleoclimatologists invoke four principal criteria. First, the material— rocks, lakes, vegetation, etc—on which the method relies must be widespread enough to provide plenty of information, since analysis of material that is rarely encountered will not permit correlation with other regions or with other periods of geological history. Second, in the process of formation, the material must have received an environmental signal that reflects a change in climate and that can be deciphered by modern physical or chemical means. Third, at least some of the material must have retained the signal unaffected by subsequent changes in the environment. Fourth, it must be possible to determine the time at which the inferred climatic conditions held. This last criterion is more easily met in dating marine sediments, because dating of only a small number of layers in a marine sequence allows the age of other layers to be estimated fairly reliably by extrapolation and interpolation. By contrast, because sedimentation is much less continuous in continental regions, estimating the age of a continental bed from the known ages of beds above and below is more risky.
    One very old method used in the investigation of past climatic conditions involves the measurement of water levels in ancient lakes. In temperate regions, there are enough lakes for correlations between them to give us a reliable picture. In arid and semiarid regions, on the other hand, the small number of lakes and the great distances between them reduce the possibilities for correlation. Moreover, since lake levels are controlled by rates of evaporation as well as by precipitation, the interpretation of such levels is ambiguous. For instance, the fact that lake levels in the semiarid southwestern United States appear to have been higher during the last ice age than they are now was at one time attributed to increased precipitation. On the basis of snowline elevations, however, it has been concluded that the climate then was not necessarily wetter than it is now, but rather that both summers and winters were cooler, resulting in reduced evaporation.
    Another problematic method is to reconstruct former climates on the basis of pollen profiles. The type of vegetation in a specific region is determined by identifying and counting the various pollen grains found there. Although the relationship between vegetation and cli- mate is not as direct as the relationship between climate and lake levels, the method often works well in the temperate zones. In arid and semiarid regions in which there is not much vegetation, however, small changes in one or a few plant types can change the picture dramatically, making accurate correlations between neighboring areas difficult to obtain.
Which of the following would be the most likely topic for a paragraph that logically continues the passage?
选项 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 invoked by paleoclimatologists when choosing a method to determine past climatic conditions.
E、A third method for investigating past climatic conditions.
答案E
解析 后接本文将会讲什么内容?看本文结构，先讲了测定古气候方法的条件，并给出两个有问题的方案。接下去应该续写方法，直到给出一个较满意的方法。∴E正确，第三种测定古气候的方法。A．“arid regions”和本文无关。B．“pollen distribution”已经在第三段提过。C．适宜保存气候变化信息的材料。第一段所讲条件内的问题。D．“other criteria”不可能，本文第一段先讲标准，再分讲各种方法，不可能再回讲标准。
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