The price of public transportation in Beijing has doubled twice since 1989, but it is still a bargain. Using the subway and minibuses used to show class status; now people of all classes take them, while some wealthy prefer taxis or private cars. What a change in just a few years! But there are downfalls to having more cars on the roads. Fortunately, the government is aware of the problem. No-lead gasoline is the only one permitted in the city, and the rest of the country follows. Thousands of trees are planted in and around the city every year. Children are taught why and how to protect the environment. At the same time, public transportation has marked real progress: buses are everywhere and run frequently. We no longer see those old buses with broken windows. Instead, there are fast buses, double-decker buses, air-conditioned or heated buses, all offering a good service.The author mainly discusses ___ of public transportation.