Reading comprehension.Subways are underground trains, which usually operate 24 hours a day. They are found in larger cities and usually run between the suburbs and the downtown area. Maps and schedules are available from the ticket offices. 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，the student union，or from the 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 as underground trains, which usually run____.

A. within downtown areas
B. away from city centers
C. in or outside big modern cities
D. between suburbs and city centers