But there are serious 4 in big cities, too. The cost of living is 5 , and there are too many people in some places of the big cities. Every years many people move to the cities because they can 6 to find work there, to study at good schools, and to receive good medical care（医疗保健）. But 7 these people can't find work or a good place to live in. Also too many people in a small space make it 8 to keep the cities 9 and clean.