A、Cows. B、Bulls. C、Cars. D、Tractors. D细节题。文中明确提到，1920年美国有2500万匹马和骡子，然而大约同时出现了马和螺子的竞争者(competitor)，那就是拖拉机(tractors)，由此可知答案为[D]。

Over the years, new technologies have changed farming. Change in a general direction is a trend. Yet people often recognize trends only when they consider the past.
    Today, we look back at some trends in American agriculture. We begin with the change from animal power to mechanical power. Our information comes from the National Agricultural Statistics Service, part of the Agriculture Department.
    In 1920, America had more than 25 million horses and mules. Most were used for farm work. Around the same time, a competitor began to appear in large numbers. Tractors could turn soil, pull loads and speed harvests — and they could do it better. More tractors meant fewer horses and mules. By the 1960s, the numbers of these work animals settled to where they remain today. That is about one-tenth the levels in 1920.
    Yet even the demand for tractors had its limits. Tractors reached their highest numbers around 1982.[30]Their numbers have been slowly decreasing. Experts say farmers can do more with less now because of new technologies. So, tractors replaced horses and mules. As a result, farmers no longer needed to raise crops to feed work animals.
    Oats have long been food for horses and mules. In 1954, American farmers planted over 16 million hectares of oats. By 2000, that was down to less than 1 million hectares.
    So what did the farmers do with the extra land? More and more farmers began to plant a new crop around the same time that the tractor became popular. It was the soybean. The soybean is one of the oldest plants harvested. Yet it was not planted widely in the United States until the 1920s.
29.What became the competitor of horses and mules in 1920?
30. Why did the number of tractors begin to decrease after 1982?
31.What did the farmers do with the extra land according to the passage?