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 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 (31) computers attached to the net-work. A socket (32) two addresses: (33) . Sockets provide a (34) communication channel between one or more systems. There are (35) sockets separately using TCP and UDP.

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】 C

【答案解析】[解析] socket套接字基本上是两个不同主机的应用程序进程之间的通信的端点。在网络上扩展的socket连接两个或者多个应用程序，这些应用运行在网路上的各个主机上。socket需要两个地址：IP地址和端口号。Socket在一个或者多个系统之间提供了全双工(full-duplex)的通信方式。有流式套接字和数据报式套接字，它们分别使用TCP和UDP。