听力原文： What was the most popular mix about the United States in the 19th Century was that of the free and simple life of the farmer. It was said that the farmers worked hard on their own land to produce whatever their families' needed. They might sometimes trade with their 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 farmers at the beginning of the 19th century. And at that time, this may have been close to the truth especially on the frontier. But by the mid century sweeping changes in agriculture were well under way as farmers began to specialized in the raising of crops such as cotton or corn or wheat. By late in the century revolutionary invents in farm machinery has vastly increased the production of specialized crops and extensive network of railroads had linked farmers throughout the country to market in the east and even overseas. By raising and selling specialized crops, farmers could afford more and finer goods and achieved much higher standard of living but at a price. Now farmers were no longer dependent just on the weather and their own efforts, their lives were increasing controlled by the banks, which had powder to grant or deny loans for new machinery, and by the railroads which set the rates for shipping their crops to the market. As businessmen, farmers now had to worry about national economic depression and the implement of world supply and demand on for example, the of price of wheat in hands. And so 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?