【听力原文】
One of the most popular myths about the United States in the 19th century was that of the free and simple life of the farmer. It was said that farmers worked hard on their own land to produce whatever their families needed.They might sometimes trade with neighbors but in general they could get along just fine by relying on themselves, not on commercial ties with others. This is how Thomas Jefferson idealized the farmer at the beginning of the 19th century. And at that time this way had been close to the truth, especially on the frontier. [13] But by the mid century, sweeping changes in agriculture were well under way as farmers began to specialize in the raising of crop such as cotton, corn or wheat. By late in the century, revolutionary advances in farm machinery had vastly increased the production of specialized crops. And the extensive network of railroads had linked farmers throughout the country to markets in the east and even overseas. By raising and selling specialized crops, farmers could afford more and find their goods, and achieved a much higher standard of living but at a price. Now farmers were no longer dependent just on the weather and their own effort. [14] Their lives were increasingly controlled by the banks which had power to grant or deny loans for new machinery and by the railroads which set the rate for shipping their crops to market. [15] As businessmen, farmers now had to worry about national economic depressions and the influence of world supply and demand on, for example, the price of wheat in Kansas. And also by the end of the 19th century, the era of Jefferson's independent farmer had come to a close.
What is the main topic of the talk?
【解题思路】四项均为名词性短语，推知本题应该是用what进行提问。四项分别为：Jefferson对于商业化农业的看法、19世纪的国际贸易、美国农业机械的改进、农民独立性的逐步丧失。本题很可能考查该听力短文的主题。
【解析】录音开头提到，19世纪初的美国农民基本上是自给自足，与外界没有太多的商业联系。到了19世纪中期。随着农民开始专业化种植作物，农业的变化发生了。l9世纪晚期，农业技术的突飞猛进极大地提高了专业化作物的生产量，密集的铁路网将农民与东部甚至海外市场联系起来，这时的农民不再仅仅依赖天气和自己的努力了，他们的生活越来越被银行和铁路控制，杰斐逊时代的独立农民也走到了尽头，因此该听力短文主要讲述的是美国农民失去独立性的过程，故选D。