Which statement describes the call behavior of the Cisco Unified Wireless IP Phone 7921G when it roams from 802.11b to 802.11a?（）

A.The phone does a directed call hangup on 802.11b，then does an auto-redial on 802.11a，creating a  new call.

B.The phone does a directed call hangup on 802.11b， then the user does a redial on 802.11a，creating a  new call.

C.The phone does a disassociation on 802.11b and an association on 802.11a，and call remains up if the  subnet has not changed.

D.The phone does not roam from 802.11b to 802.11a when on an active call.

E.The phone must switch network profiles to roam from 802.11b to 802.11a.