单项选择题 Which layer of the OSI reference model is responsible for ensuring reliable end-to-end delivery of data?（）

单项选择题 You download a file from an FTP site on the Internet. What is the highest layer in the OSI model used in this FTP operation?（）

单项选择题 While troubleshooting a connectivity problem on the network, you issue the ping command from your PC command prompt, but the output shows "request times out."At which OSI layer is this problem associated with?（）