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
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