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

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

A. After upgrading the standby main control board first, the upgrade of the original active main control board can be completed as long as the main control board is switched through A.

B. Upgrade the active main control board first, and then upgrade the standby main control board

C. Complete the upgrade of the standby main control board first, then perform a hard reset on the current active main control board to make it active and standby switchover, and then upgrade the current active main control board

D. First upgrade the active main control board and then upgrade the standby main control board in the same way

Answer: C