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.
