too many cars have created a lot of serious problems in our world. besides congestion, accidents and fast fuel consumption, cars are responsible for a good part of air pollution in big cities.

　　all the time, they are pumping huge amounts of waste gases into the atmosphere. these gases are very harmful, causing disease and even death. one possible solution is to design and develop clean cars and clean fuels. in shanghai, some of the public buses begin to run on natural gas, which does not give off as much carbon dioxide as the petrol.

　　but it may take decades for the new models of clean cars completely replace the traditional ones. another solution is to develop modern public transportation systems and restrict the use of private cars. if the price of petrol rises constantly and the public vehicles are efficient and convenient enough, most people will not buy private cars.

　　and the total number of cars in big cities will reduce greatly. on the whole, the elimination of air pollution needs the collective efforts from the government, the public and the environmentalists.