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