单项选择题
Which one of the following is the most commonly used layer 2 network device?（）
A. Hub B. Bridge C. Switch D. Router E. Repeaters F. None of the above
多项选择题 In the OSI model, at which layers do WANs operate at?（）
单项选择题 At which layer of the OSI model is the optimal path to a network destination determined at?（）
多项选择题 In the communications industry, what are the features and benefits of using the layered OSI model?（）
