多项选择题 In the communications industry, what are the features and benefits of using the layered OSI model?（）

单项选择题 Which of the protocols below, operates at Layer 2 of the OSI model, and is used to maintain a loop-free network?（）

单项选择题 Which OSI layer is associated with the following: The acknowledgement of transmissions, sequencing, and flow control across a network?（）