题目内容：
根据短文内容，回答题。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 areas．Maps and schedules are available from the ticket office．If yon take the subway often，you can save money by purchasing a monthly pass(月票)．
City operatedbuses run on various routes(线路)and are designed t。be at certain places at certain time．Maps and schedules may be posted at certain stops，or they may be available at local banks．1ibraries，the student union，or from the bus drivers．Buses run mainly during the day. Fareis paid by exact change in coins，or by monthly passes.
Taxis are generally m。re expensive in the United States than in other countries. If You use a taxi，be sure y。u ask the amount of the fare before you agree toride．The driver usually expects a tip(小费)of l5 percent of the fare．
According to the passage，subways are 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
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