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.