New York, London, Paris and other big cities are exciting places to live in. There are many interesting things to see and to do. You can go to different kinds of museums, plays and films. You can also go shopping 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 of big cities. Every year many people move to the cities because there are some 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 in. 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. Which is the best title(题目)for this passage

A. Big cities.
B. New York, London, and Paris.
C. Exciting Places in Big Cities.
D. Serious Problems in Big Cities.