In a point-to-point WDM networking scenario, the signal-to-noise ratio of a single channel is low, while other channels are normal. What is the reason for this being impossible?

In a point-to-point WDM networking scenario, the signal-to-noise ratio of a single channel is low, while other channels are normal. What is the reason for this being impossible?
A . The optical power of the OTU board corresponding to this wave at the transmitting end has decreased
B . The fiber pigtail attenuation from the OTU board corresponding to this wave to the combiner increases
C . Increased attenuation on the optical path
D . The gain slope of the amplifier changes

Answer: C