What is required for the Barge feature to work?

What is required for the Barge feature to work?

A.
A conference bridge

B.
A shared line

C.
A conference already in process

D.
A multiple line appearance

Explanation:
Barge and Privacy
The Barge and Privacy features work with each other. Both features work with only shared lines.
http://www.cisco.com/en/US/partner/products/sw/voicesw/ps556/products_administration_guide_chapter09186 a