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.