What caused this message on a Cisco 9971 IP phone, connected to a Cisco 3750X PoE switch, when a mobile phone is plugged into the IP Phone’s back USB port?

Refer to the exhibit.

What caused this message on a Cisco 9971 IP phone, connected to a Cisco 3750X PoE switch, when a mobile phone is plugged into the IP Phone’s back USB port?
A. The back USB port only supports Cisco USB devices such as a Cisco Unified Video camera
B. USB classes for this USB port are not properly configured
C. The USB port is not enabled by the administrator
D. The mobile phone is requesting more power than the USB port could provide
E. USB devices are not supported when the IP phone is powered by a Cisco PoE switch