单项选择题

Over half the world’s people now live in cities. The latest "Global Report on Human Settlements" says a significant change took place last year. The report 31 this week from U. N. Habitat, a United Nations agency.
A century ago, 32 than five percent of all people lived in cities. 33 the middle of this century it could be seventy percent, or 34 six and a half billion people.
Already three-fourths of people in 35 countries live in cities. Now most urban population 36 is in the developing world.
Urbanization can lead to social and economic progress, but als0 put 37 on cities to provide housing and 38 . The new report says almost two hundred thousand people move 39 cities and towns each day. It says worsening inequalities, 40 by social divisions and differences in 41 , could result in violence and crime unless cities plan better.
Another issue is urban sprawl (无序展的城区). This is where cities 42 quickly into rural areas, sometimes 43 a much faster rate than urban population growth.
Sprawl is common in the United States. Americans move a lot. In a recent study, Art Hall at the University of Kansas found that people are moving away from the 44 cities to smaller ones. He sees a 45 toward "de-urbanization" across the nation.