托福阅读：句子插入题的三种逻辑

1. 转折关系
一旦给定句中出现转折关系，就意味着该句的前后文内容存在反义。如下例：
Leatherbacks keep their body heat in three different ways. The first, and simplest, is size. The bigger the animal is, the lower its surface-to-volume ratio; for every ounce of body mass, there is proportionately less surface through which heat can escape. An adult leatherback is twice the size of the biggest cheloniid sea turtles and will therefore take longer to cool off. Maintaining a high body temperature through sheer bulk is called gigantothermy. ■ It works for elephants, for whales, and, perhaps, it worked for many of the larger dinosaurs. ■ It apparently works, in a smaller way, for some other sea turtles. ■ Large loggerhead and green turtles can maintain their body temperature at a degree or two above that of the surrounding water, and gigantothermy is probably the way they do it. ■ Muscular activity helps, too, and an actively swimming green turtle may be 7 °C (12.6 °F) warmer than the waters it swims through.
However, these animals have additional means of staying warm.
从给定句的语义不难看出，该句后文必然要提到某种保暖的方法。而从however引导的转折逻辑和additional可以想到，这是一种“额外”的 方法，也就意味着在该句前文已经提到过另一种保暖方法。通读原文，可以发现只有空格4符合要求（前有gigantothermy，后有muscular activity）。

2. 因果关系
一旦给定句中出现因果关系，就意味着该句是原文中某句的原因（一般在原文句前）或结果（一般在原文句后）。如下例：
■ Another task for the Glomar Challenger’s scientists was to try to determine the origin of the domelike masses buried deep beneath the Mediterranean seafloor. ■ These structures had been detected years earlier by echo-sounding instruments, but they had never been penetrated in the course of drilling. ■ Were they salt domes such as are common along the United States Gulf Coast, and if so, why should there have been so much solid crystalline salt beneath the floor of the Mediterranean? ■
Thus, scientists had information about the shape of the domes but not about their chemical composition and origin.
从给定句的语义不难看出，该句描述的是某种科学研究方法，能让科学家了解到dome的形状，但无法获知其化学成分和起源，而这种方法的出现应该在该句 前面。通读全文，不难发现空格3前提及曾使用echo-sounding instruments（回声探测设备），但没有经过drilling（钻探），完全符合要求。

3. 递进关系
与转折和因果相比，递进是一种许多考生都感觉比较陌生的逻辑关系。它表示的是后文在前文基础上在语义上更加深入。但无论如何深入变化，后文也无法脱离 前文的基础，也就是说前后文的主题应保持基本一致，这点和并列关系类似。所以在句插题的给定句中一旦出现递进关系，就意味着该句的关键词必然能在原文中某 处找到复现，从而插入复现句子后面的空格。如下例：
Transportation was becoming less of a problem for those who wished to move west and for those who had farm surpluses to send to market. ■Prior to 1815, western farmers who did not live on navigable waterways were connected to them only by dirt roads and mountain trails. ■Livestock could be driven across the mountains, but the cost of transporting bulky grains in this fashion was several times greater than their value in eastern markets. ■The first step toward an improvement of western transportation was the construction of turnpikes. ■These roads made possible a reduction in transportation costs and thus stimulated the commercialization of agriculture along their routes.
In fact, goods could be shipped more cheaply across the much greater distance of the Atlantic Ocean than they could from western New York to coastal cities.
不难发现，if fact就是一个典型的递进连词。该句中提及more cheaply，所以其前句必然也会出现和价格或成本相关的内容。通读全文，可见空格3前居中出现cost of transporting，符合要求。