Which of the following operates as a SIP back-to back user agent?

Which of the following operates as a SIP back-to back user agent?

A.
Cisco TelePresence codec

B.
Cisco TelePresence Multipoint Switch

C.
Cisco Unified Cornmunic3tions Manager

D.
SIP gateway that is used to access the PSTN

E.
H.323 gateway