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.
注意喂奶的姿势，母乳喂养时应让婴儿的嘴唇完全含住（）。
A. 乳头
B. 乳头和部分乳晕
C. 乳头和大部分乳晕
D. 以上都是
资源的回收与再利用是人类保护环境的重要途径。下列哪一人物的创新体现了这一点（）
A. 蔡伦
B. 张仲景
C. 祖冲之
D. 华佗
站用交流电源系统竣工（预）验收（站用交流电源柜）标准卡中，对站用交流电源柜竣工（预）验收外观检查有哪些验收内容？
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