In the OSNR model for EDFA amplifiers， SNR OUT = 1/（1/SNR IN + FhvB/P IN ），  what does SNR IN represent？（） A:  the signal to noise of the current optical amplifier B:  the signal to noise of the previous amplifier or source C:  the signal to noise introduced by the optical multiplexer D:  the accumulated noise to signal ratio of the optical channel