Which statement correctly describes the Cisco implementation of RSTP？ （）
- APortFast， UplinkFast， and BackboneFast specific configurations are ignored in Rapid PVST mode.
- BRSTP is enabled globally and uses existing STP configuration.
- CRoot and alternative ports transition immediately to the forwarding state.
- DConvergence is improved by using sub-second timers for the blocking， listening， learning， and forwarding port states.