We moved slowly through the city in a taxi and entered a crowded slum district. The temperature was 　1　over 100 and the air was thick with dust and smoke. The streets seemed alive with people. People eating, people washing, people sleeping. People visiting each other, arguing and screaming. People pushing their hands 　2　the taxi windows, begging. People relieving themselves. People holding 　3　the sides of buses. People leading animals. People, people, people. As we drove slowly through the crowd, 　4　the taxi's horn, the dust heat, noise and cooking fires made it 　5　 a scene from Hell. Would we ever get to our hotel? All three of us were, I 　6　, frightened. Since that night, I've known what overpopulation 　7　like.
　　Statistics show that rapid population growth 　8　problems for developing countries. So why don't people have fewer children? Statistics from the developed countries suggest that it 　9　 only when people's living standards begin to rise 　10　birth rates begin to fall. There are good reasons 　11　this. Poor countries cannot 　12　social services and old age pensions, and people's incomes are so low they have nothing to 　13　for savings. 　14　, many people look 　15　their children to, in their old age, 　16　them with security. Having a large family can be a form of insurance. And even while they are still quite young, children can do a lot of useful jobs on a small farm. So poor people in a developing country will need to see clear 　17　of much better conditions ahead 　18　they will think of having smaller families. But their conditions cannot be improved 　19　there is a reduction in the rate at which population is increasing. This will depend on a very much wider acceptance of family planning and this, 　20　, will mean basic changes in attitudes.
______

A.across 

B.in 

C.over 

D.through