The fact that the worlds cities are getting more and more crowded is a well-documented demographic fact. Cities such as Tokyo, Sao Paolo, Bombay and Shanghai are now considered 'mega-cities', reflecting their enormous size and huge populations, which in many cases is around the twenty million-mark. There are two main reasons why these and other cities are becoming so crowded: one economic and other socio-cultural.【M1】______ Undoubtedly, the primary cause for cities becoming so crowded is economic. With a country develops, its cities become【M2】______ the engines of development. Furthermore, many jobs are available【M3】______ in these areas. Tokyo, for example, is the motor for Japans rapid【M4】______ economic development in the 1960 s and 70 s; as a result, its population increased rapidly. People moved to Tokyo so that they could find jobs and establish economic security themselves and【M5】______ their families. A further factor in the huge increase in urban populations is the socio-cultural factor. Thousands of people migrating to the【M6】______ cities not only for jobs but also for educational and personnel【M7】______ reasons. The better universities are always located in big cities and this attracts tens of thousands of students every year. These students usually stay up and work in the city after they graduate.【M8】______ Moreover, young people will move to the city as if the villages and【M9】______ rural areas are often governed by custom and tradition, which they believe is an obstacle to their private freedom. In conclusion, we can say that amongst the many causes behind urban population density, the economic or cultural are the【M10】______ most significant. People always move to the areas which provide opportunity and the freedom they desire. It is now the responsibility of municipal councils to make sure these people are housed, clothed, fed and educated. 【M1】