问题：

[单选] Which statement correctly describes the Cisco implementation of RSTP？ （）

A . PortFast， UplinkFast， and BackboneFast specific configurations are ignored in Rapid PVST mode.
B . RSTP is enabled globally and uses existing STP configuration.
C . Root and alternative ports transition immediately to the forwarding state.
D . Convergence is improved by using sub-second timers for the blocking， listening， learning， and forwarding port states.