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 control board directly, and then upgrade the standby main control board
C . Complete the upgrade of the standby main control board first, and then perform a hard reset of the current active main control board to make its active and standby switchover, and then upgrade the current compensation standby main control board.
D . Upgrade the main control board first and then upgrade the standby main control board in the same way

Answer: C