托福阅读推断题：逆向推理

托福阅读中的推断题其实是事实信息题的加强版，其所有的推断都是基于原文事实信息的逻辑推断，也就是说我们所作出的推断要建立在读懂文章意思的基础上的。既然如此，事实信息题的做法可以迁移到推断题上——找词定位，逻辑推断，匹配答案。

找词定位这一步我们在事实信息题中已经阐述得很清楚了在这里不再赘述。这里重点要阐述的是推断题中最常考的逆向推断。
　　逆向逻辑类似于“非”的关系。比如，2014年寒假之后，小明开始准备托福考试了。这里蕴含的逆向逻辑推理是：2014年寒假之前，小明还没开始准备托福考试。TPO 20 Westward Migration中第1题考得就是上述逆向逻辑推理：

What can be inferred from paragraph 1 about western farmers prior to 1815?
　　O They had limited their crop production to wheat, corn, tobacco, and cotton.
　　O They were able to sell their produce at high prices.
　　O They had not been successful in raising cattle.
　　O They did not operate in a national market economy.

根据题干中的数字1815，我们定位至原文中After 1815 improved transportation enabled more and more western farmers to escape a self-sufficient way of life and enter a national market economy.这句话——“1815年之后，改善后的交通使得更多的西部农民摆脱自给自足的生活方式并参与到国有市场经济中来”。那么也就是说，1815年之前，西部农民是没有摆脱自给自足的生活方式并没有参与到国有市场经济中。所以答案选第三个选项。

当然不是每个逆向推理都只是简单的时间上的逆向推断，还有意义上的逆向推断，如TPO 12 Water in the Desert 中第1题：

2. Which of the following statements about annual rainfall can be inferred from paragraph 1?
　　○ Flat desert areas receive more annual rainfall than desert areas with mountains.
　　○ Areas that receive more than four inches of rain per year are not considered deserts.
　　○ Many areas receive less than four inches of annual rainfall, but only a few are deserts.
　　○ Annual rainfall has no impact on the groundwater resources of desert areas.

Paragraph 1:
　　Rainfall is not completely absent in desert areas, but it is highly variable. An annual rainfall of four inches is often used to define the limits of a desert. The impact of rainfall upon the surface water and groundwater resources of the desert is greatly influenced by landforms. Flats and depressions where water can collect are common features, but they make up only a small part of the landscape.

利用题干中主题词annual rainfall回原文定位第二句“An annual rainfall of four inches is often used to define the limits of a desert.”意思是：年降水量为四英寸定义了沙漠的界限。很明显，利用反义推断，年降水量低于四英寸是沙漠，那么高于四英寸的肯定就是非沙漠。故选项3为正确答案。

类似的逆向逻辑在托福阅读中比比皆是，比如TPO 12 Transition on Sound in Films 第5题、TPO-17 Europe’s Early Sea Trade with Asia 中第6题等等，大家可以利用此法实际演练一下。

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