—If you have a backache, you should lie down and rest.

—____________      [ ]

A. That's too bad                  

B. Sorry

C. You're welcome                    

D. That's a good idea

在STP中，某网桥发现拓扑改变，它发出的拓扑改变通知消息的端口是（）

A.阻塞端口

B.指定端口

C.根端口

D.所有端口