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 farmers. 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 was how Thomas Jefferson idealized the farmers at the beginning of the 19th century. And at that time, this might have been close to the truth especially on the frontier. But by the mid century sweeping changes in agriculture were well underway as farmers began to specialize in raising crops such as cotton or corn or wheat. By late in the century, revolutionary advances in farm machinery have vastly increased production of specialized crops and an extensive network of railroads had linked farmers throughout the country to markets in the east. By raising and selling specialized crops, farmers could afford more and finer goods and achieve a much higher standard of living but at a price. Now farmers are no longer dependent just on the weather and their own effort, their lives are increasingly controlled by banks, which have the 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 have to worry about national economic depressions and the influence of world supply and demand on, for example, the price of wheat in Kansas. 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？

A. Jefferson's views about commercialized agriculture.
B. International trade in the nineteenth century.
C. Improvements in farm machinery in the United States.
D. Farmers' loss of independence.