Question 25 is based on the passage you have just heard.

听力原文：

    Do you know where a mule comes from? It is the child of a donkey and a horse. （22）Mules have strong muscles like horses, but they eat less, can work longer, and are gentler, like donkeys.

    George Washington was the first person in the United States to own mules. He had heard that mules made good farm animals and he contacted the U.S. ambassador in Spain to ask about them.（23）In 1785, King Charles III of Spain sent Washington a male donkey as a gift. That male donkey became the father of the mule industry in the U.S.

    Every April, Maury County holds a Mule Day celebration. Held in Columbia, Tennessee,（24）Mule Day had its beginnings as “Breeder’s Day” in the 1840s. Farmers and farm animal breeders would bring their animals to market every April to show, buy, and trade. This was an important business before the days of tractors, when many families made a living from farming and mules were used as work animals.（25）Eventually, tractors began to replace mules, making them less in demand.

    A parade was added to Mule Day in 1934 to attract more people. Over the years other activities have been added, and today more than 200,000 people show up each year to watch and participate. If you visit during Mule Day celebrations, you might see a mule-driving contests, a square dances, horse shows or even tree-cutting competitions.

25. What made mule less in demand in America?

解析：B。本题目为细节题。问题为：什么使得美国队骡子的需求量下降？根据原文“Eventually, tractors began to replace mules, making them less in demand.”，可知最终拖拉机开始代替骡子，这使得骡子的需求量减少。故正确答案为B）The arrival of tractors（拖拉机的出现）。