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【单选题】
安装电灯线路时（）。
A.
开关必须安装在火线上
B.
灯螺纹必须安装在火线上
C.
不作明确要求
D.
灯口内中心的弹簧片必须安装在零线上
参考答案：
参考解析：
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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