单项选择题 At which of the following layers does the TCP protocol operate?（）

单项选择题 A user on a network cannot access any network resources. The link light on the computer showsthat a connection is present. Which of the following is the MOST likely cause of the problem?（）

单项选择题 A company wants to secure access to its internal wireless network. The company wants to use themost secure means to access the network. Which of the following is the BEST choice for wireless security in this situation?（）