【U nit4背景材料】现代社会,公共交通系统已相当发达。人们为了提高效率,出门办事时,不管路程远近,一般很少走路,即使自己没有车,也可以借助公共交通工具,十分方便。在英美等西方发达国家,拥有私家车的人很多,但不少市民仍然选择搭乘公共汽车、地铁等。公共汽车作为一种最普通 [U nit4 background material] In the modern society, the public transportation system is quite developed. When people go out to work in order to improve their efficiency, they rarely walk, no matter how far they are. Even if they do not have a car, they can use public transportation, which is very convenient. In western developed countries such as Britain and the United States, there are many people who own private cars, but many citizens still choose to take buses, subways, etc. Buses are the most common