托福閱讀：句子插入題的三種邏輯

1. 轉折關系
一旦給定句中出現轉折關系，就意味著該句的前后文內容存在反義。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.
從給定句的語義不難看出，該句后文必然要提到某種保暖的方法。而從however引導的轉折邏輯和additional可以想到，這是一種“額外”的 方法，也就意味著在該句前文已經提到過另一種保暖方法。通讀原文，可以發(fā)現只有空格4符合要求（前有gigantothermy，后有muscular activity）。

2. 因果關系
一旦給定句中出現因果關系，就意味著該句是原文中某句的原因（一般在原文句前）或結果（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.
從給定句的語義不難看出，該句描述的是某種科學研究方法，能讓科學家了解到dome的形狀，但無法獲知其化學成分和起源，而這種方法的出現應該在該句 前面。通讀全文，不難發(fā)現空格3前提及曾使用echo-sounding instruments（回聲探測設備），但沒有經過drilling（鉆探），完全符合要求。

3. 遞進關系
與轉折和因果相比，遞進是一種許多考生都感覺比較陌生的邏輯關系。它表示的是后文在前文基礎上在語義上更加深入。但無論如何深入變化，后文也無法脫離 前文的基礎，也就是說前后文的主題應保持基本一致，這點和并列關系類似。所以在句插題的給定句中一旦出現遞進關系，就意味著該句的關鍵詞必然能在原文中某 處找到復現，從而插入復現句子后面的空格。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.
不難發(fā)現，if fact就是一個典型的遞進連詞。該句中提及more cheaply，所以其前句必然也會出現和價格或成本相關的內容。通讀全文，可見空格3前居中出現cost of transporting，符合要求。