Agriculture is the number one industry in the United States and agricultural products are the country's leading export. American farmers manage to feed not only the total population of the United States, but also millions of other people throughout the rest of the world. Corn and soybean exports alone account for approximately 75 percent of the amount sold in world markets. This productivity, however, has its price. Intensive cultivation exposes the earth to the damaging forces of nature. Every year wind and water remove tons of rich soil from the nation's croplands. Each field is covered by a limited amount of topsoil, the upper layer of earth which is richest in the nutrients and minerals necessary for growing crops. Ever since the first farmers arrived in the Midwest almost 200 years ago，cultivation and, consequently, erosion have been decreasing the supply of topsoil. In the 1830s, nearly two feet of rich, black top soil covered the Midwest. Today the average depth is only eight inches, and every decade another inch is blown or washed away. This erosion is steadily decreasing the productivity of valuable cropland. A United States Agricultural Department survey states that if erosion continues at its present rate, corn and soybean yields in the Midwest may drop as much as 30 percent over the next 50 years. So far, farmers have been able to compensate for the loss of fertile topsoil by applying more chemical fertilizers to their fields; however, while this practice has increased crop yields, it has been devastating for ecology. Agriculture has become one of the biggest polluters of the nation's precious water supply. Rivers, lakes, and underground reserves of water are being filled in and poisoned by soil and chemicals carried by drainage from eroding fields. Furthermore, fertilizers only replenish the soils they do not prevent its loss. The last sentence in the first paragraph gives an example to show_______.

单项选择题The closest restatement of the last sentence in,the 3rd paragraph is______.

单项选择题In the 2nd paragraph 'roads' means_______.

单项选择题Happiness can be described as a positive mood and a pleasant state of mind. According to recent polls (民意测验) sixty to seventy percent of Americans consider themselves to be moderately happy and one in twenty persons feels very unhappy. Psychologists have been studying the factors that contribute to happiness. It is not predictable nor is a person in an apparently ideal situation necessarily happy. The ideal situation may have little to do with his actual feelings. A good education and income are usually considered necessary for happiness. Though both may contribute, they are only chief factors if the person is seriously undereducated or actually suffering from lack of physical needs. The rich are not likely to be happier than the middle-income group or even those with very low incomes. People with college educations are somewhat happier than those who did not graduate from high school, and it is believed that this is mainly because they have more opportunity to control their lives. Yet people with a high income and a college education may be less happy than those with the same income and no college education. Poor health does not rule out happiness except for the severely disabled or those in pain. Learning to cope with a health problem can contribute to happiness. Those with a good sex life are happier in general, but those who have a loving, affectionate relationship are happier than those who rely on sex alone. Love has a higher correlation with happiness than any other factor. It should be noted that people quickly get used to what they have, and they are happiest when they feel they are increasing their level no matter where it stands at a given time. Children whose parents were happily married have happier childhoods are not necessarily happier adults. The best formula for happiness is to be able to develop the ability to tolerate frustration, to have a personal involvement and commitment, and to develop self-confidence and self-esteem. It can be inferred from the passage that______.

单项选择题To check population growth is necessary when_______.

单项选择题At the present rate, approximately how many years later the black top soil now covering the Midwest will completely be blown or washed away?

单项选择题The world has become a world of cities. With the present rate of urban growth(3. 8% in the Third World) , the majority of the population of the world will be living in cities by the year 2000. This will transform the rural-urban equation which has marked the history of mankind up to now and will call for new example and a great deal of innovation to face this phenomenon. This being so, one must accept the fact that for some years to come, no policy will be capable of stopping or reversing the present migratory trend from the rural areas to the cities in the Third World. In Africa, the urban population will reach 330 million people by the end of the century as against 150 million in 1995. The number of people living in shanty-towns (贫富窟) will inevitably increase in spite of the efforts to improve housing conditions. Africa alone needs to build 12 million housing units between now and the year 2000 to meet its most basic needs. In an ILO study, M. S. V. Sethuraman estimates that in 70 Third 'World cities the proportion of people living in shanty-towns varies from 15% to 70% and that about US $ 116 billion is required to give minimum comfort to these people by the turn of the century—less than US $ 10 billion per year. The world population is growing at a rate of about 90 million people per year, with the Third World accounting for 80 million of them. The pressure on cities can only go on increasing. The urban population of the developing countries will exceed 2 billion people by the year 2000 and since the main reason for the high demographic (人口统计的) growth is poverty, the additional population will be mostly made of people of very limited means. If the urban population of the developing countries exceeds 2 billion people by the year 2000, the main problem the additional people will face is______.land

单项选择题Newspapers often tell us of floods in some parts of the United States. Nearly every year on the great central drainages heavy rains and melting snow cause the waters to pour out the mountains and plains, to turn brooks into torrents, and to swell quiet streams into wild uncontrolled rivers. From Cairo to New Orleans, and from Pittsburgh to Paducah, the cry 'River rising!' is a familiar yet fearful voice. . . When the rivers sometimes become too high or too swift to be controlled communities are flooded, families flee from their homes, croplands are washed out, and transportation comes to a halt. Hunger, disease, and death follow the wild waters. Although given less publicity, the agricultural damage done by the many smaller, more frequent floods usually far exceeds the losses caused by the very grand ones. In the Central States, ditches and drains cause the flows from spring rains and melting snow to run far more rapidly than in the days before white men settled on the land. Once, excess spring flood waters emptied into lakes and swampy lands, there to be detained for slow release into stream and rivers. Now, systematic drainage has actually eliminated these natural reservoirs. In the more rolling sections of the East, spring runoff was formerly absorbed and held temporarily in the porous soils beneath the unbroken expanse of forest. When large areas were converted to farm use, removal of the forest and the practice of up-and-down hill plowing deprived the soils of much of their ability to catch and store water. The effects of eliminating the natural forest cover are shown in the gullied farm lands and widened stream channels found in some densely settled areas. Partly because the stream channels are more or less filled with material washed down from the uplands, and partly because storm runoff has increased, the channels are today no longer able to carry all the flow from heavy rains. This explains why the streams overtop banks far more often than in the days before settlement. The best title for the selection would be______.

单项选择题All of the following cause floods EXCEPT______.

单项选择题In order to compensate for the loss of fertile topsoil, farmers have been_______.

单项选择题All of the following are statements about the disadvantages of fertilizers EXCEPT that________.