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．sending

B．routing

C．switching

D．receiving

除去二氧化碳中混有少量一氧化碳的方法是

A．将一氧化碳倒出　　

B．将气体点燃

C．将气体通过足量澄清石灰水中

D．将气体通过灼热的氧化铜