What is a key configuration requirement for Hybrid Message Service High Availability deployment with multiple IM and Presence clusters?

What is a key configuration requirement for Hybrid Message Service High Availability deployment with multiple IM and Presence clusters?
A . Your IM and Presence Service clusters must have Multiple Device Messaging disabled.
B . AXL service should be activated only on the publisher of each IM and Presence cluster
C . You must have the Inter cluster Lookup Service working across all of your IM and Presence clusters
D . You must have the Inter cluster Sync Agent working across your IM and Presence clusters

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Answer: D