A transport layer protocol has several responsibilities. One is to create a process-to-process (program-to-program) communication; TCP uses port （） to accomplish this. Another responsibility of a transport layer protocol is to create a （） and error-control mechanism at the transport level. TCP uses a sliding （） protocol to achieve flow control. It uses the acknowledgment packet, time-out, and retransmission to achieve （） control. The transport layer is also responsible for providing a connection mechanism for the application program. The application program sends （） 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.

The application program sends （） of data to the transport layer.

A．ports

B．streams

C． packets

D．cells

传输控制TCP协议和用户数据报UDP协议是互联网传输层的主要协议。下面关于TCP和UDP的说法中，______是不正确的

A．TCP是面向连接的协议，UDP协议是无连接的协议
B．TCP能够保证数据包到达目的地不错序，UDP不保证数据的传输正确
C．TCP协议传输数据包的速度一般比UDP协议传输速度快
D．TCP保证数包传输的正确性，UDP在传输过程中可能存在丢包现象