1. [单选题]SMTP工作在OSI哪一层（）
A. 会话层
B. 表示层
C. 传输层
D. 应用层
2. [单选题]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?
A. The data link layer
B. The application layer
C. The access layer
D. The session layer
E. The network layer
