北半球出现极昼极夜现象的最南界是[ ]
A．北极圈
B．北极点
C．南极圈
D．南极点
答案：A
They often do sports, ______ swimming, running and playing football.
A．as well as
B．as usual
C．as a result
D．such as
行灯的电源电压不得超过（）V。
A.12
B.36
C.48
D.60
