Which statement best describes Cisco supervisor engine redundancy using Stateful Switchover？（）
- ASwitchover ensures that Layer 2 through Layer 4 traffic is not interrupted
- BRedundancy requires BGP， OSPF， EIGRP， or IS-IS
- CRedundancy provides fast supervisor switchover for all Cisco Catalyst 6500 series switches
- DSwitchover can be caused by clock synchronization failure between supervisor