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little to sell or to trade for other goods. Subsistence farms were common in the United States at one time, but there are few such farms today. Two thirds of American farmers practice commercial farming. Commercial farms produce about 97 per cent of the country''s agricultural goods. These farms are operated much like other industries. The members of the farm family produce the commodities（商品） that will be sent to the market. They sell their products and buy the things they need. Some commercial farms are big factories in the field. They may be owned by a company or by an individual as an investment. Most American farms, however, are still operated by families.A farmer may own or rent his land. In a country as large as the United States, farms vary widely in climate, land surface, soil,water supply, convenience to market, and other characteristics. Different regions may have different kinds of specialized farms. Specialized farmers concentrate on the kind of farming best suited to the land and climate, to their technical skills, and to the money they invest in the farm. However, no region is limited to a single type of farm. Indeed few farms are limited to one crop or commodity. Where output is varied and no one kind of crop or commodity produces more than half of a farm''s income, it is classified as a general farm. The general farmer grows varied crops to use the different kinds of soils or surfaces on his land and to employ his time and machinery most efficiently. Subsistence farmers______ A.grow crops for their families'' own useB.sell a lot of their products for moneyC.trade their varied output for other goodsD.use their agricultural products as commodities 参考答案： A grow crops for their families'' own use----------------------------------------7、 Subsistence farms______. A.are very popular in the United States nowB.are still common in many parts of the worldC.consist of two thirds of the total number of the farms in the world todayD.used to be the major form of farming in the world 参考答案： B are still common in many parts of the world----------------------------------------8、 American commercial farms______. A.furnish the country with two thirds of the food its people needB.are run in much the same way as other industriesC.are all owned by different companiesD.never rent their land for farming 参考答案： B are run in much the same way as other industries----------------------------------------9、 A specialized farm is characterized by ______. A.its especially large sizeB.its independence of the marketC.its concentration on a single type of farmingD.its special way of operation 参考答案： C its concentration on a single type of farming----------------------------------------10、 A farm is classified as general if ______. A.more than half of its income comes from varied cropsB.it is limited to a single type of farmC.it employs different kinds of machinesD.the soils or surfaces of its land are used most efficiently 参考答案： A more than half of its income comes from varied crops----------------------------------------11、Passage Three No one knows for sure why birds migrate（定期迁徙）。 One theory of migration says that ancient birds of the northern part of the earth were forced southward during the Ice Age, when ice covered large parts of Europe, Asia and North America. As the ice melted, the birds came back to their homelands, spent the summer, and then went south again in winter. Gradually, these comings and goings became habits, and birds now migrate though much of the ice has gone. Another theory proposes that the ancie
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