When Cisco Unified Communications Manager is in a default configuration, why are all calls possible for all calling sources?

When Cisco Unified Communications Manager is in a default configuration, why are all calls possible for all calling sources?

A.
Because all entities are placed in the <None> partition and in the <Default> CSS.

B.
Because all entities are placed in the <Default> partition and the <Default> CSS.

C.
Because entities that do not have a CSS assigned can only access calling targets without a partition assigned.

D.
Because any calling target can only reach those calling sources that are in the <Default> CSS.