Beijing, Shanghai, Guangzhou and other big cities are great places to live in. There are many interesting things to see and do. You can go to different kinds of museums, see all kinds of plays and films. You can also buy things from all over the world. But there are serious problems in big cities, too. It's expensive to live there, and there are too many people in some places of big cities. Every student can study at good schools and receive good medical care . But sometimes these people can't find work or good places to live in. Also it is hard to keep the cities safe and clean. Some people enjoy living in big cities, others do not. Before they move to a big city, they should think about the problems living there. In this passage the writer thinks it's right for people   _  .
to move to big cities not to move to big cities. to move to big cities without thinking. to think of the problems before living in big cities.