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参考答案：
参考解析：
刷刷题刷刷变学霸
举一反三
【单选题】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 area. Maps and schedules are available from t...
A.
within downtown areas
B.
away from city centers
C.
in or outside big modern cities
D.
between suburbs and city centers
【单选题】所谓“中线”是指（）。
A.
通过上切牙中间缝隙的一条直线
B.
通过下切牙中间缝隙的一条直线
C.
将颅面部左右等分的一条假想线
D.
通过上唇系带的一条直线
E.
通过下唇系带的一条直线