Which two best practices should be utilized?

Northern Trail Outfitters has determined they will initially deploy messaging to email and SMS channels, but also plan for social advertising. Customer data originates within their point-of-sale system which communicates to Marketing Cloud in real-time via API. They want to configure their customer data for long-term cleanliness and maintainability.

Which two best practices should be utilized? (Choose 2 answers)
A . Configure Contact Builder to automatically generate a unique subscriber key.
B . Define the subscriber key as a unique value that does not relate to a specific channel.
C . Regularly merge duplicate contacts to keep tracking data accurately.
D . When using an external database of record, utilize that system’s identifier as the contact key.

Answer: C,D