问题：

[单选] 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 vla