单项选择题
HowdoestheEthernetRelayServiceusetheVLANtag？（）
A.toprovideserviceinternetworking
B.tosupporttransparencyforLayer2frames
C.asaconnectionidentifiertoindicatedestination
D.asamappingtotheDLCIinserviceinternetworking
E.toprovideatrunkbywhichallVLANscannavigatefromonesitetooneormultiplesites