托福阅读例题1：
The extreme seriousness of desertification results from the vast areas of land and the tremendous numbers of people affected, as well as from the great difficulty of reversing or even slowing the process.
A. Desertification is a significant problem because it is so hard to reverse and affects large areas of land and great numbers of people.
B. Slowing down the process of desertification is difficult because of population growth that has spread over large areas of land.
C. The spread of deserts is considered a very serious problem that can be solved only if large numbers of people in various countries are involved in the effort.
D. Desertification is extremely hard to reverse unless the population is reduced in the vast areas affected.
逻辑分析：
原句带有因果逻辑词result from，result from前面部分“the extremeseriousness of desertification”属于结果，result from后面部分属于原因。
根据优先看相同逻辑的选项，我们优先分析带有because的A和B选项：
B 选项，原属于原因部分的“slowingdown the process”出现在了该选项的结果部分，所以因果颠倒，可排除。
A 选项，结果部分和原句意思一致，都是表示沙漠化问题严重，原因部分也和原句一致，都是说明了两个具体原因，答案选A。
托福阅读例题2：
Third, in a species-rich ecosystem, diseases may spread more slowly because most species will be relatively less abundant, thus increasing the average distance between individuals of the same species and hampering disease transmission among individuals.
A. In any ecosystem, as the number of individuals in the same species increases, the rate of disease transmission slows down.
B. Ecosystems that have a small number of different species tend to be disease-free, because the species’ habitats are at a safe distance from each other.
C. In ecosystems with many species, diseases spread more slowly because there are fewer individuals in a species and, as a result, the individuals are more widely scattered.
D. The average distance between individuals in a species-rich ecosystem increases, so diseases are prevented from being communicated between species.
逻辑分析：
这题属于逻辑关系套逻辑关系，整个句子是因果逻辑，信号词为because。
在because之前的为结果，意思是在物种丰富的生态系统中，疾病的蔓延会相对慢一些。
在because之后的原因部分中又套了一个小的因果逻辑，意思是大多数物种相对不那么丰富，因此(thus)导致了两个结果，其一是相同物种之前隔的平均距离就远了，其二是阻碍了个体物种之间的疾病传播。
转化为结构图，可以理解为：
优先分析逻辑关系一致的选项，得出了BCD：
B 选项，结果部分意思是数量少的物种所在的生态系统没有疾病，而原因部分的结构和信息完全被打乱，与原文不一致，可排除。
C 选项，与原句一样，是大因果逻辑关系套着小因果逻辑关系。结果部分和原文一致，because之后原因部分先讲了小原因，即有更少有的物种存在，和原文less abundant一致，小结果部分概括了两个并列结果。 逻辑、结构、语义与原文一致，答案选C。
D 选项，原因部分指出物种平均距离增加，此信息出现在原句大因果逻辑关系中的小原因，选项原因部分缺少一个衔接的逻辑关系，可排除。
D选项中缺少的逻辑关系如何理解呢?举个例子：
今天下雨了，所以我上班迟到了。
这句话中文看来，逻辑似乎没有问题，但是如果放在英语中，尤其是严谨的科学说明中，这句话是不成立的。不是下雨导致我迟到，而是下雨导致交通堵塞，才导致我迟到。