光纤直放站中继端机要完成（）转换（下行）和（）转换（上行）。
A、电/光
B、光/电
C、光/光
D、电/电
参考答案：A, B
The song was very popular _______.[ ]
A. in the 1990
B. in 1990s
C. in the 1990s
D. on 1990s
类序
