单项选择题 According to the OSI model, which of the following layers is responsible for providing reliablecommunication?（）

单项选择题 A technician receives a call on monday morning that several computers will not connect to the network. The technician determines that the computers left on over the weekend are functional and those that were turned off over the weekend are not functioning. Which of the following is the MOST likely source of the problem?（）

单项选择题 A remote user cannot connect to the office via VPN. The user has established Internet connectivity. Which of the following is the MOST likely next step in troubleshooting?（）