题目内容：
根据下列题干，回答题：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 （ 1 ）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 separate computers at tached to the network．A socket（ 2 ）two addresses：（ 3 ） ．Sockets provide a（ 4 ） communication channel between one or more systems．
There are stream and datagram sockets separately using TCP and（ 5 ） ．
请选择（1）处最佳答案：
参考答案：
答案解析：