小安阅读 26的一个问题，没有被问过

典座

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 snow-line 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 climate 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.




5. The author discusses lake levels in the southwestern United States in order to

(A) illustrate the mechanics of the relationship between lake level, evaporation, and precipitation
(B) provide an example of the uncertainty 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


答案是B，我选了D。根据前面那句“Moreover, since lake levels are controlled by rates of evaporation as well as by precipitation, the interpretation of such levels is ambiguous. ”我觉得后面的lake levels in the southwestern United States是为了解释这句啊......虽然之前在BD之间犹豫了好久，但是还是采坑了。这种题有没有大侠有神马技巧啊，很多篇文章都是完全读的懂，但是做题狂采坑。

Strakk

觉得B还是不错的。问题问的是，后面那一句话的目的/意义。B意思说作为前一句话的一个example，没问题。
D的逻辑误区个人觉得主要在于：“Moreover, since lake levels are controlled by rates of evaporation as well as by precipitation, the interpretation of such levels is ambiguous. ”----这是一个example。它仅可以用来 suggest ONE of the situations for which effects of increased rates of evaporation on levels of precipitation，但却不能用来说它 EXPLAIN the effect of increased rates of evaporation on levels of precipitation

典座

Strakk 发表于 2020-7-31 07:25
觉得B还是不错的。问题问的是，后面那一句话的目的/意义。B意思说作为前一句话的一个example，没问题。
D的 ...

你的意思是lake levels in the southwestern United States只解释了一部分，所以D错在不严谨是吗？
谢谢解答啦