When joining a data extension in Contact Builder, in which scenario should a consultant mark the Use as Root checkbox?

When joining a data extension in Contact Builder, in which scenario should a consultant mark the Use as Root checkbox?
A . Supplemental data for an interaction in Journey Builder.
B . Database of record other than All Subscribers.
C . One-to- Many Relationship to the Contact Record.
D . Data for both Email and MobileConnect sends.

Answer: B