接下来，出国留学网阅读栏目小编要和大家详解的是托福阅读句子插入题的本质和原则，为了避免在细节上失分，和出国留学网小编一起来了解一下吧~
PART1：一个本质---插入句是一句“可有可无”的话。
本题的出题位置大家比较清楚，一般处于倒数第二题的位置，并且用来出题的段落是大家应该在前面的题目中已经见到过，而之前的题目和文章既然大家理解起来没有出现大的障碍，就说明插入进来的句子是----一句可有可无的话。这就要从两个方面来理解了。
何为“可有”?
既然插入句能放进原文中去，就说明它和原文是有一定关系的，所以叫“可有”，这也就意味着，插入句的部分含义是在原文中有所重复的。所以我们的重点是回到文中找那部分被重复的句意。例如：
Watt’s steam engine soon showed what it could do。 It liberated industryfrom dependence on running water。 The engine eliminated water in the mines bydriving efficient pumps， which made possible deeper and deeper mining。 Theready availability of coal inspired William Murdoch during the 1790s to developthe first new form of nighttime illumination to be discovered in a millenniumand a half。 Coal gas rivaled smoky oil lamps and flickering candles， and earlyin the new century， well-to-do Londoners grew accustomed to gaslit houses andeven streets。 Iron manufacturers， which had starved for fuel while depending oncharcoal， also benefited from ever-increasing supplies of coal： blast furnaceswith steam-powered bellows turned out more iron and steel for the new machinery.Steam became the motive force of the Industrial Revolution as coal and iron orewere the raw materials。
(TPO6-Powering the Industrial Revolution)
Look at the four squares that indicate where the following sentencecould be added to the passage。
The factories did not have to go to the streams when power could co...