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______.