Refer to the exhibit.  What is the effect when the switchport priority extend cos 3 command is  configured on the switch port interface connected to the IP phone？（）

• A、 The IP phone is enabled to override with aCoS value of 3 the existing CoS marking of the PC  attached to the IP phone.
• B、 The switch will no longer tag incoming voice packets and will extend the trust boundary to the  distribution layer switch.
• C、 RTP will be used to negotiate aCoS value based upon bandwidth utilization on the link.  
• D、 Effectively， the trust boundary has been moved to the PC attached to the IP phone.
• E、 The computer is now establishing theCoS value and has effectively become the trust boundary.