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问题 选择题

下列图示中的水，可以看作纯净物的是

A．河湖水

B．温泉水

C．蒸馏水

D．自来水

答案

答案：C

题目分析：纯净物是由一种物质组成的物质，混合物是由多种物质组成的物质，据此分析。

A、河湖水中含有水和多种可溶性及不溶性杂质，属于混合物，故错误；

B、温泉水中含有水和多种可溶性及不溶性杂质，属于混合物，故错误；

C、蒸馏水中只含有一种物质——水，属于纯净物，故正确；

D、自来水中含有水和多种可溶性杂质，属于混合物，故错误。故选C

点评：解答本题要充分理解纯净物和混合物的区别，要分析物质是由几种物质组成的，如果只有一种物质组成就属于纯净物，如果有多种物质就属于混合物。

单项选择题

Throughout the 19th century and into the 20th, citizens of the United States maintained a bias against big cities. Most lived on farms and in small towns and believed cities to be centres of (1) , crime, poverty and moral (2) Their distrust was caused, (3) , by a national ideology that (4) farming the greatest occupation and rural living (5) to urban living. This attitude (6) even as the number of urban dwellers increased and cities became an essential (7) of the national landscape. Gradually, economic reality overcame ideology. Thousands (8) the precarious (不稳定的) life on the farm for more secure and better paying jobs in the city. But when these people (9) from the countryside, they carried their fears and suspicions with them. These new urbanities, already convinced that cities were (10) with great problems, eagerly (11) the progressive reforms that promised to bring order out of the (12) of the city.

One of many reforms came (13) the area of public utilities. Water and sewerage systems were usually operated by (14) governments, but the gas and electric networks were privately owned. Reformers feared that the privately owned utility companies would (15) exorbitant (过渡的) rates for these essential services and (16) them only to people who could afford them. Some city and state governments responded by (17) the utility companies, but a number of cities began to supply these services themselves. (18) of these reforms argued that public ownership and regulation would (19) widespread access to these utilities and guarantee a (20) price.

19（）

A.secure

B.ensure

C.reassure

D.incur
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多项选择题

在下列哪些情况下不允许打开反推整流罩（）

A.发动机大功率下

B.风速大于40节

C.前缘襟翼放下

D.发动机慢车下
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