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 oncomputers attached to the network. A sockettwo addresses:. Sockets provide acommunication channel between one or more systems.
There aresockets separately using TCP and UDP.

A．E-mail address and IP address

B．MAC address and port address

C．MAC address and IP address

D．port number and IP address

交感神经节后纤维的递质是（）

A.乙酰胆碱

B.去甲肾上腺素

C.5-羟色胺

D.多巴胺

E.去甲肾上腺素或乙酰胆碱