In the following essay, each blank has fou
题型：单项选择题
问题：
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.
（34）是（）
A.full-duplex
B.half-duplex
C.simplex
D.complex