Beijing, Shanghai, Guangzhou and other big cities are exciting places to live. There are many interesting things to see and to do. You can go to different kinds of museums, plays and movies. You can also go to supermarkets to buy things from all over the world. But there are serious problems in big cities, too. The cost of living is high, and there are too many people in some big cities. Every year many people move to the cities because there are more chances to find jobs, to study at good schools, and to receive good medical care. But sometimes these people cannot find work or a good place to live. Also, too many people in a small space make it hard to keep the cities safe and clean. Some people enjoy living in big cities. Others do not. Before people move to big cities, they should think about the problems of living there. According to the passage, big cities  _
are not exciting places to live have not many serious problems are terrible are not always clean and safe enough