Sure! Here are the suggested answers for the blanks in the text:

21. **B. overcrowded**
22. **A. living**
23. **C. so**
24. **D. suburbs**
25. **A. leave**

So, the completed sentences would read:

Today, people all over the world are moving out of small villages in the country to go and live in big, noisy cities. They are moving from the peaceful mountains, fields and rivers of the countryside to the busy world of streets, buildings, traffic and **overcrowded**. This movement from rural areas to urban areas has been going on for over two hundred years. In many countries, the main reason why people come to live in towns and cities is work. After one or two large factories have been built in a town, people come to find work and a better **living**, and soon an industrial area begins to grow. There is usually a residential area nearby where the factory workers can live. The families of these workers need schools, shops and hospitals, **so** more people come to live in the area to provide these services - and so a city grows. A lot of people live in the **suburbs** of the city, far away from the industrial area and the city centre. Some suburbs are pleasant, with nice houses and parks for children to play, but others are poor, with old houses and bad conditions. But what is the future of the big cities? Will they continue to get bigger and bigger? Perhaps yes. Some major cities have actually become bigger in the last ten years and it is quite possible that one day we will see people **leave** the countryside and go to bigger towns and cities.