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