【正确答案】 D

【答案解析】[听力原文] What was 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 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. But by the mid-century sweeping changes in agriculture were well under way as farmers began to specialize in the raising of crops such as corn or wheat. By late in the century revolutionary inventions in farm machinery had vastly increased the production of specialized crops and extensive network of railroads had linked farmers throughout the country to the markets in the East and even overseas. By raising and selling specialized crops, farmers could afford more and more finer goods and achieve much higher standards of living but at a price. Now farmers were no longer dependent just on the weather and their own efforts. 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 rates for shipping their crops to the market. As businessmen, farmers now had to worry about national economic depression and the influence of world supply and demand on, for example, the price of wheat in Kansas. And so, by the end of the 19th century, the Jefferson's era of the independent farmer had come to a close. What was the major change in agriculture during the 19th century?