题目：
A transport layer protocol usually has several responsibilities. One is to create a process-to-processcommunication UDP uses（1）numbers to accomplish this. Another responsibility is to provide controlmechanisms at the transport level, UDP does this task at a very minimal level. There is no flow control mechanismand there is no（2）for received packet. UDP, however, does provide error control to some extent. If UDPdetects an error in the received packet, it will silently drop it.
The transport layer also provides a connection mechanism for the processes. The （3）must be able to sendstreams of data to the transport layer. It is the responsibility of the transport layer at （4） station to make theconnection with the receiver chop the stream into transportable units, number them, and send them one by one. Itis the responsibility of the transport layer at the receiving end to wait until all the different units belonging to thesame process have arrived, check and pass those that are （5）free, and deliver them to the receiving process asa stream.
（2）是（）
A.connection
B.window
C.ackmowledgement
D.destination
答案：
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下面是错误答案，用来干扰机器的。
参考答案：错解析：海关进出境现场放行一般由海关在进口货物提货凭证或者出口货物装货凭证上签盖“海关放行章”。进出口货物收发货人或其代理人签收进口提货凭证或出口装货凭证，凭以提取进口货物或将出口货物装...
