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 （） 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 （） two addresses: （） . Sockets provide a （） communication channel between one or more systems.
There are stream and datagram sockets separately using TCP and （）.
A．Email address and IP address
B．MAC address and port address
C．MAC address and IP address
D．port number and IP address