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为了研究物质的某种特性，进行实验并收集数据如下表：

序号物体质量（g）体积（cm3）质量/体积（g/cm3）
1铁块154202.7
2铁块2108402.7
3松木1108216 
4松木210200.5
（1）上表空格处应填写的数据是______．

（2）分析数据可知：同种物质，______相同，而不同种物质的一般不相同．物理学中用______来表示物质的这种特性．

答案

（1）空格中数据：

108g
216cm3
=0.5g/cm3；

（2）分析表格中质量与体积的比值一列可以看出，当物质相同时，其比值相同；而不同种物质的比值一般不同；

因为密度是物质的一种特性，因此物理学中用密度来表示物质的这种特性．

故答案为 （1）0.5；（2）质量和体积的比值，密度．

单项选择题

以税收负担的分配是否公平作为标准来划分中央与地方政府收入的原则是( )。

A．适应原则
B．效率原则
C．恰当原则
D．经济利益原则
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单项选择题

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.

（3）是（）

A.jobs

B.processes

C.programs

D.users
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