单项选择题
Subways are underground trains, which usually operate 24 hours a day. They are found in larger cities and usually run between suburbs and downtown areas. Maps and schedules are available from the ticket office. If you take the subway often, you can save money by purchasing a monthly pass(月票). City-operated buses run on various routes(线路)and are designed to be at certain places at certain times. Maps and schedules may be posted at certain stops, or they may be available at local banks, libraries, student unions, or from bus drivers. Buses run mainly during the day. Fare is paid by exact change in coins, or by monthly passes. Taxis are generally more expensive in the United States than in other countries. If you use a taxi, be sure you ask the amount of the fare before you agree to ride. The driver usually expects a tip(小费)of 15 percent of the fare.According to the passage, subways are underground trains, which usually run______.
A.within the downtown areas
B.away from the city centers
C.in or outside the big modern cities
D.between the suburbs and the city centers