A、About 1920. B、Around 1925. C、Around 1930. D、About 1935. A短文中提到，1920年，美国用于农业生产的马、骡数量有2500多万，“同时一个竞争者开始大量出现。拖拉机……”意即1920年拖拉机开始大量

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. [22]In 1920, America had more than 25 million horses and mules. Most were used for farm work. [22]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.
    [23]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. Their numbers have been slowly decreasing since 1982. Experts say farmers can do more with less now because of new technologies. As tractors replaced horses and mules, 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 sixteen million hectares of oats. By 2000, that was down to less than one 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. By the year 2000, close to 30 million hectares were planted with soybeans. [24]It is the nation’s most important crop for high-protein animal feed and for vegetable oil. [25]In fact, soybeans are the second most valuable crop grown by American farmers after corn. Much of the soybean production goes to exports.
22. In which year did tractors begin to appear in large numbers?
23. How many horses and mules did America have in the 1960s?
24. What is America’s most important crop for high-protein animal feed and for vegetable oil?
25. What is the most valuable crop grown by American farmers?