Which of the following is an error in the system OSNR commissioning?

Which of the following is an error in the system OSNR commissioning?
A . Adjust the single wave group first, and then adjust the main line optical power
B . One of the principles for adjusting the OSNR flatness of the MPIR point system: After the system power is determined, the OSNR flatness of the MPI-R point is tested by a spectrometer. If the OSNR flatness does not meet the standard (OSNR flatness index s21B), it needs to be adjusted.
C . The combiner is an M40V board. "Look at the receiving end and adjust the transmitting end". While ensuring that the optical power flatness meets the index requirements, the optical power flatness can be sacrificed to ensure the flatness of OSNR (or BER).
D . APE automatically adjusts the optical power of each channel to ensure the flat optical power at the receiving end, thereby ensuring the flatness of OSNR

Answer: A