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