For NG-SDH equipment equipped with dual main control, when the main control board is upgraded, which of the following statement is correct?

For NG-SDH equipment equipped with dual main control, when the main control board is upgraded, which of the following statement is correct?A . After completing the upgrade of the standby main control board, the original active main control can be upgraded if the standby main control board is switched.B . Upgrade the active main...