题目内容：
根据下面的文字资料回答题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 computer sat tached to the network．Asocket（ 2 ）twoaddresses：（ 3 ）．Sockets providea （ 4 ）communication channel between one or more systems.There are stream and datagram sockets separately using TCP and（ 5 ）．文中（ 1 ）处正确的答案是（ ）。
A．program B．UDP
C．socket
D．TCP
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