A transport layer protocol has several responsibilities, One is to create a process-to-process (program-to-program) communication; TCP uses port (11) to accomplish this. Another responsibility of a transport layer protocol is to create a (12) and error-control mechanism at the transport level．TCP uses a sliding (13) protocol to achieve flow control．It uses the acknowledgment packet，time-out，and retransmission to achieve (14) control．The transport layer is also responsible for providing a connection mechanism for the application program．The application program sends (15) of data to the transport layer．It is the responsibility of the transport layer at the sending station to make a connection with the receiver．
A．ports
B．streams
C．packets
D．cells
参考答案：B
解析：[参考译文] 传输层协议有多个职责。一是创建进程到进程(程序到程序)的通信，由TCP使用端口号来完成；另一个职责是在传输层创建流和差错控制机制。TCP利用滑动窗口协议来实现流控制。它使用确认分组、超时、重传机制来完成差错控制。传输层也负责为应用程序提供连接机制，应用程序将数据流发送到传输层，发送端的传输层负责建立于接收端之间的连接。