Which option describes a function of SIP preconditions?

A.
SIP preconditions enable end-to-end RSVP over an SIP trunk.

B.
SIP preconditions enable RSVP between Cisco IP Phones.

C.
SIP preconditions can be enabled in a gatekeeper.

D.
SIP preconditions enable end-to-end RSVP for calls through the PSTN.