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________.

单项选择题The floods which are given most publicity______.

单项选择题According to the selection, streams overtop their banks partly because______.

单项选择题By 'infrastructure'(Para. 2) the author means_______.

单项选择题The future population will be older than today, and this in turn will change the patterns of social demands. However, Mexico will still be far from the 'aging societies' that will most likely prevail (占优势 ) in the industrialized countries. In less than 25 years, the country will have to add almost as much infrastructure as it has already built up to now, simply to maintain services and production at the current levels. This will be a tremendous challenge, although a similar one was faced successfully in the recent past, when available infrastructure was doubled in two decades or so. It is no wonder that much has been said about the need to reduce or preferably halt Mexico's population control which is taken for granted as both good and necessary. It has also gained supporters in the developing world, and Mexico is no exception. But the arguments about population are complicated, as the following discussion illustrates. Even if Mexico's population reaches 125 million by the year 2010, its population density is still smaller than the 1985 population density of some 50 countries. By international standards Mexico will still not be overpopulated by the year 2010. If wealth is generated by people, the more individuals there are, the greater the wealth that potentially could be generated. Why should Mexico control its population at density levels below those of the richer countries if more population represents the possibility of generating more wealth? On the other hand, it is often pointed out that once sustainability(支撑力) limits are near or are reached, there are decreasing productivity gains (or,perhaps more accurately, increasing productive losses) , and people become a cost rather than an asset. If we assume that there are sustainability limits and that we are close to reaching these limits at a world level, then it is appropriate to check population growth. According to the passage, the population argument is focused on_______.