Milan, the second largest city of Italy, has won an undesirable world record. Of the 54 big cities studied by the World Health Organization and the United Nations, Milan is considered to have the worst air pollution. According to these two10-year studies, the level of air pollution in Milan is twice as high as the levels in Paris and Beijing and three times those in Tokyo and London, and four times that in New York. The reason for Milan's bad pollution, to some experts, is that the city lies in a valley that holds back the poisonous gases. Though Milan has taken some measures to lower the level of pollution, the situation is still worrying. People joke to each other, "If you are traveling to Milan, be sure to bring your gas mask." In order to reduce the use of private cars, in 1979, the government in Milan began an interesting experiment: passengers in the city buses did not have to pay for their tickets. Why is the air pollution in Milan so serious?
Because too many cars are being driven in the city every day. Because the city lies in a place where there are some mountains near it. Because Milan has taken some measures to encourage people to buy private cars. Because the city lies in a place where it is difficult for polluted air to move away.