Which statement about vlan operation on cisco catalyst switches is true?（）
A. when a packet is received from an 802.1Q trunk,the vlan id can be determined from the source MAC address table.
B. unkown unicast frames are retransmitted only to the ports that belong to the same vlan
C. ports between switches should be configured in access mode so that vlans can span across the ports
D. broadcast and multicast frames are retransmitted to ports that are configured on different vlan.
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