G.652光纤可以将2.5Gbit/s速率的信号无电再生中继传输至少（）公里左右。
A、200
B、400
C、600
D、800
参考答案：C
自动增益控制
用适当的介词填空。
l. China has many world records _____ its beautiful buildings and structures.
2. He has been a teacher _____ he was 18.
3. Mount Tai is located _____ Shandong Province.
4. He has walked _____ two hours.
5. The project will be finished_____2020.
