Which statement about SIP precondition is most correct?

Which statement about SIP precondition is most correct?

A.
When configuring SIP precondition, the SIP trunk must have access to an RSVP agent.

B.
When configuring SIP precondition, the IP phones must have access to an RSVP agent.

C.
When configuring SIP precondition, the IP phones and SIP trunk must have access to an RSVP agent.

D.
RSVP agents are only required for the IP phones. SIP trunks require RSVP agents only when fall back to local RSVP is configured.

E.
SIP trunk will always require RSVP agents regardless of what RSVP type is configured.

Explanation: