Which sharing setting would allow users to collaborate with their peers who work for the same partner firm, but not with any users outside of their firm?

Universal Containers is implementing a Partner Community.

Which sharing setting would allow users to collaborate with their peers who work for the same partner firm, but not with any users outside of their firm?
A . Community User Visibility
B . Chatter Group Member Visibility
C . Site User Visibility
D . Portal User Visibility

Answer: A