In a wavelength division system, after a certain distance of transmission, the optical signal-to-noise ratio of the receiving end signal will be reduced to a certain extent. The main reason is ？

In a wavelength division system, after a certain distance of transmission, the optical signal-to-noise ratio of the receiving end signal will be reduced to a certain extent. The main reason is ？
A . The fiber attenuation is too large
B . Use of demultiplexer
C . Use of EDFA
D . No new optical fiber is used

Answer: C