Which two of these are characteristics of the 802.1Q protocol?（）

A. it is a layer 2 messaging protocol which maintains vlan configurations across network.
B. it includes an 8-bit field which specifies the priority of a frame.
C. it is used exclusively for tagging vlan frames and dose not address network reconvergence following switched network topology changes.
D. it modifies the 802.3 frame header,and thus requires that the FCS be recomputed.
E. it is a trunking protocol capable of carring untagged frames.