题目内容
（请给出正确答案）
以下关于port trunking和spanning tree说法正确的是：
A、生成树技术是一个纯备份的技术，在应用的时候有一条或多条链路处在阻塞(blocking)状态
B、生成树技术只有在主链路断掉以后，备份链路才会启动
C、端口聚合提供了扩展带宽、链路热备、均衡负载和快速切换(一般小于4秒)的特性
D、相比较而言，生成树技术投资较大，而端口聚合技术较便宜，但是浪费链路带宽
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A.not good
B.unsafe
C.safe
D.obtainable
A.Cargo Manifest
B.Certified Crew List
C.Curio List
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A．a city in the mountains
B．a city on the Coast
C．a city with many people
D．a country on the coast
A．the two port cities were destroyed by huge earthquakes
B．the disappearance of the two port cities was caused by underwater landslide
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