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.