Which statement describes the call behavior of the Cisco Unified Wireless IP Phone 7921G when it roams from 802.11b to 802.11a?（）
- AThe phone does a directed call hangup on 802.11b，then does an auto-redial on 802.11a，creating a new call.
- BThe phone does a directed call hangup on 802.11b， then the user does a redial on 802.11a，creating a new call.
- CThe phone does a disassociation on 802.11b and an association on 802.11a，and call remains up if the subnet has not changed.
- DThe phone does not roam from 802.11b to 802.11a when on an active call.
- EThe phone must switch network profiles to roam from 802.11b to 802.11a.