[单选题]SMTP工作在OSI哪一层（）

正确答案 ：D

应用层

[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?

正确答案 ：E

The network layer

解析：解析：Explanation:

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