单项选择题
A.Layer 1
B.Layer 3
C.Layer 4
D.Layer 5
单项选择题 Which of the following security devices would MOST likely be used on a network to screen and prevent malicious traffic?（）
单项选择题 Which of the following is an intelligent repeater that is aware of the MAC addresses of the nodeson either side of the data link?（）
单项选择题 Which of the following WAN technologies uses a telephone cable?（）