单项选择题

In the following essay, each blank has four choices. Choose the most suitable one from the four choices and write down in the answer sheet.
A socket is basically an end point of a communication link between two applications. Sockets that extend over a network connect two or more applications running on (1) computers attached to the network. A socket (2) two addresses: (3) . Sockets provide a (4) communication channel between one or more systems.
There are (5) sockets separately using TCP and UDP.

4（）

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