Which configuration concept allows for high-availability on IM and Presence services in a UC environment?

Which configuration concept allows for high-availability on IM and Presence services in a UC environment?
A . IM and Presence subclusters (configured on Cisco UCM)
B . Presence Redundancy Groups (configured on Cisco Unified IM and Presence)
C . IM and Presence subclusters (configured on Cisco Unified IM and Presence)
D . Presence Redundancy Groups (configured on Cisco UCM)

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Answer: D