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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 statements best describes the organization of the passage as a whole?
选项 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 contradictory data.
E、The author describes how methods for determining past climatic conditions were first developed and then describes two of the earliest known methods.
答案C
解析 全文组织结构，主要内容：A．描述一种方法，然后给出例子。文中并不是对一具体方法给多个例证。B．“date”不是全文目的，而是考察古气候的一个步骤。C．正确。先给出测定古气候方法要满足的条件(第一段)再给出几个方法的实例(后两段)。D．“contradictory data”原文未提。E．“first developed”和“earliest”原文均未提。
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