问题：

[单选]

A customer has 4 POWER6 servers managed by a Hardware Management Console （HMC）. They plan to install a second HMC for redundancy. 
Which of the following statements correctlydescribes the dual HMC environment？ （）

A .  To provide the Service Agent function， both HMCs must have Service Agent enabled and a dedicated modem and phone line.
B .  Authorized users must have the same username and password on both HMCs， so that every user of one HMC is automatically authorized on the other.
C .  Both HMCs are automatically notified of any changes that occur in the managed systems， so results of changes made to the managed system by one HMC are visible on the
D .  Both HMCs must be running the same software revision. Software maintenance must be performed on both HMCs at the same time， but one HMC should be disconnected from