During the nineteenth and twentieth centuries, migrations have taken place within 11 countries; the cities with their industries have attracted people away from the country. The possibility of earning a fixed 12 in a factory or office was more attractive than the possibility of staying on the farm and having one's work 13 by frost, storms, or droughts. Furthermore, the development of agricultural machinery made it possible for fewer people to do the same 14 of work.

　　Thus, at the same time when the industrial revolution made it possible to produce goods more 15 and more quickly in factories, agricultural revolution also took place. Instead of leaving fields empty every third year, farmers began to plant clover or some other crop that would 16 the soil. Instead of using only animal fertilizer, farmers began to use chemical fertilizers to keep the soil

　　rich. These methods have enabled French farmers, for example, to get five times as much wheat as was 17 from the same land two centuries ago.

　　In many countries farmers find it more 18 to raise only one crop or one kind of animal. They choose the kind that gives the best results. Then they sell all that they produce, instead of trying to grow a little of everything and consume what they grow. This is a more feasible type of 19 because modern methods and machinery are adapted to specific animals and specific crops. Therefore, it would be too expensive to do all the work by hand, or to buy the 20 needed for several different kinds of farming. (责任编辑：admin)