Which statement best describes Cisco supervisor engine redundancy using Stateful Switchover？（）

• A、Switchover ensures that Layer 2 through Layer 4 traffic is not interrupted
• B、Redundancy requires BGP， OSPF， EIGRP， or IS-IS
• C、Redundancy provides fast supervisor switchover for all Cisco Catalyst 6500 series switches
• D、Switchover can be caused by clock synchronization failure between supervisors