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问题 选择题

自秦朝历汉唐至明清的两千余年间，中国古代国家的特征是 

①皇帝“家天下” ②中央集权制度 ③官僚制度 ④“多民族”与“大一统”

A．①②

B．①④

C．①②③

D．①②③④

答案

答案：D

不定项选择题

既能跟盐酸反应又能跟NaOH溶液反应的化合物是[ ]

A．Mg

B．Al2O3

C．Na2CO3

D．NaHCO3

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单项选择题

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．hop

B．port

C．route

D．packet
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