[单项选择]{{I}}Questions 18 to 20 are based on the following passage. At the end of the passage, you will be given 15 seconds to answer the questions. Now listen to the passage.{{/I}}What’s the talk mainly about A. The advantages of an economy based on farming. B. Reasons farmers continued using river transportation. C. The role of cotton in the United States economy. D. Improved methods of transporting farm crops.