Beijing is the capital of China. It is also one of the birthplaces of the Chinese civilization   and one of the six ancient capital cities in China. It is the second biggest city with an area of 17,020 square kilometers. Beijing is changing every day and it is becoming more and more beautiful. Now there are a lot of new buildings, modern shopping malls and large markets. In the past, Beijing was very clean and quiet and most people went to work or school by bike. Things have changed a lot. Now people can take the bus, the underground or the taxi to go to work or school. It is fast and convenient. The changes in Beijing have brought many advantages but they have also caused some problems: air pollution, noise pollution, sandstorms and lots of traffic. How many kinds of problems are mentioned in this passage?
four five six seven