A transport layer protocol usually has several responsibilities．One is to create a process-to-process communication；UDP uses (71) numbers to accomplish this．Another responsibility is to providecontrol mechanisms at the transport level．UDP does this task at a very minimal level．There is no flowcontrol mechanism and there is no (72) for received packet．UDP, however,does provide errorcontrol to some extent．If UDP detects an error in the received packet，it will silently drop it．
The transport layer also provides a connection mechanism for the processes．The (73) must be able to send streams of data to the transport layer．It is the responsibility of the transport layer at (74) station to make the connection with the receiver,chop the stream into transportable units，numberthem，and send them one by one．It is the responsibility of the transport layer at the receiving end to waituntil all the different units belonging to the same process have arrived，check and pass those that are (75) flee，and deliver them to the receiving process as a stream．

A．connection

B．window

C．acknowledgement

D．destination

根据汉语提示完成句子, 每空一词。

1.  下了一整天的雨,我只好呆在家里看光盘.

        It ______ all day, I had to stay at home and_____DVDs.

2.  我需要帮助,你能把土豆切碎么?

    I _____ some help. Can you ____up the tomatoes, please?

3. 昨天,我帮露西买了块新手表.

    I ________Lucy_______ a new watch yesterday.

4. 我在上周的歌唱比赛里获得了第一名

    I _____the first prize in the _____competition last week.

5. 玛丽是个有趣的女孩,她总是让我们笑个不停.

    Mary is a funny girl, she always____us_____.