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

单项选择题 Which of the following ports does POP3 services use, by default?（）

单项选择题 A user took a laptop on vacation and made changes to the configuration in order to use the device at the hotel. The user can reach the Internet, but cannot access any internal network resources. Which of the following is the MOST likely reason?（）