Nowadays，millions of migrant workers flock into cities in search ofjobs and better living．However,with the sharp rise in the urban population，many problems arise in the development of cities． Firstly，cities become more and more crowded，putting much pressure upon transportation，housing，sanitation， education，employment and SO on．City services and facilities have been strained to a breaking point．Secondly，a growing number of private cars emit a huge amount of carbon dioxide，leaving the air mercilessly polluted．What is more，the city is also threatened by an increase in crime．There is not a single day passing without the report of someone being robbed，kidnapped or even murdered． Last but not least，city—dwellers are not only separated from the nature but also isolated from each other，even not knowing the name of their next—door neighbor． All these problems have harmed the attractiveness of the city．More and more people may seek to live in the suburbs if there isn’t any improvement．
