To initiate testing， 
which port does a Cisco IOS IP SLA source use to send a control message to an IP SLA responder？ （）

A.required at the destination to implement Cisco IOS IP SLA services
B.improves measurement accuracy
C.required for VoIP jitter measurements
D.provides security on Cisco IOS IP SLA messages via LEAP or EAP-FAST authentication
E.responds to one Cisco IOS IP SLA operation per port
F.stores the resulting test statistics

A.only implemented between Cisco source and destination-capable devices
B.statistics provided by syslog， CLI， and SNMP
C.measures delay， jitter， packet loss， and voice quality
D.only monitors VoIP traffic flows
E.provides active monitoring
F.provides passive monitoring

A.Test major changes before deployment and defer minor changes until during deployment. 
B.Document and verify rollback procedures.
C.Ensure consistency of code versions across the network.
D.Progressively modify procedures and documentation during implementation.