Nobody really knows how big Lagos is. What’s indisputable is that it’s growing very quickly. Between now and 2050, the urban population of Africa could triple. Yet cities in sub-Saharan Africa are not getting richer the way cities in the rest of the world have. Most urban Africans live in slum (贫民窟); migrants are often not much better off than they were in the countryside. Why?

    The immediate problem is poverty. Most of Africa is urbanizing at a lower level of income than other regions of the world did. That means there’s little money around for investment that would make cities liveable and more productive. Without upgrades and new capacity, bridges, roads and power systems are unable to cope with expanding populations. With the exception of South Africa, the only light rail metro system in sub-Saharan Africa is in Addis Ababa, Ethiopia. Traffic jam leads to expense and unpredictability, things that keep investors away.

In other parts of the world, increasing agricultural productivity and industrialization went together. More productive farmers meant there was a surplus that could feed cities; in turn, that created a pool of labour for factories. But African cities are different. They are too often built around consuming natural resources. Government is concentrated in capitals, so is the money. Most urban Africans work for a small minority of the rich, who tend to be involved in either cronyish (有裙带关系的) businesses or politics. Since African agriculture is still broadly unproductive, food is imported, consuming a portion of revenue.

    So what can be done? Though African counties are poor, not all African cities are. In Lagos, foreign oil workers can pay as much as $65,000 per year in rent for a modest apartment in a safe part of town. If that income were better taxed, it might provide the revenue for better infrastructure. If city leaders were more accountable to their residents, they might favour projects designed to help them more. Yet even as new roads are built, new people arrive. When a city’s population grows by 5% a year, it is difficult to keep up.

46. B) They are growing fast without becoming richer.

解析：首先在题目中找到定位词cities in sub-Africa，结合选项回原文定位至第1段第3，4两句。定位句指出，从现在起到2050年，非洲城市人口可能会增加到原来的三倍；而撒哈拉以南的非洲城市却没有像世界上其它地区的城市变得富有。最后看选项：A) 它们有比世界上其他城市更多的贫民窟。第一段最后只是提到大多数的市民住在贫民窟，并没有强调贫民窟的数量之多，也不涉及与其他城市的对比故排除。B) 它们发展的很快却并没有变的富裕，符合定位句信息，故正确。C) 它们和其他的城市一样现代化，和定位句意义不符，故排除。D) 它们吸引了那些想变得更富有的移居者。定位句最后提到移居者并没有比他们住在乡下时变得更富有，故排除。