单选题 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 (66) 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 (67) two addresses: (68) . Sockets provide a (69) communication channel between one or more systems.
There are stream and datagram sockets separately using TCP and (70) .

A、 Email address and IP address
B、 MAC address and port address
C、 MAC address and IP address
D、 port number and IP address

【正确答案】 B

【答案解析】[解析] Socket是两个应用之间的联系接口。Socket使网络中不同的计算机之间的应用能够联系在一起。一个Socket有两个地址：端口号和IP地址。Socket提供了在多个系统中全双工通信的方式。TCP在 Socket传递的是字符流，而UDP传递的是数据报。