多项选择题
Which three of the protocols below belong to the application layer?（）
A.ARP B.HTTPS C.SMTP D.CDP E.TFTP F.ICMP
多项选择题 Which three of the following OSI model layers also belong to the TCP/IP model?（）
单项选择题 Which one of the following is the most commonly used layer 2 network device?（）
多项选择题 In the OSI model, at which layers do WANs operate at?（）
