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问题 选择题

常温时，下列各溶液中，离子的物质的量浓度关系正确的是　

A．饱和碳酸钠溶液中：c(Na+)= 2c(CO32－)+ 2c(HCO3－)+2c(H2CO3)

B．pH=2的醋酸中：c(H+)=0．02mol•L－1

C．0．1 mol•L－1氯化铵溶液中：c(H+)> c(OH－)> c(Cl－)> c(NH4+)

D．pH=12的NaOH溶液与等体积0．01 mol·L－1氯化铵溶液混合后所得溶液中：c(Na+) = c(NH4+)

答案

答案：A

题目分析：A、在碳酸钠溶液中，根据物料守恒，钠元素的浓度是碳元素的2倍，碳元素的存在形式有CO32－、HCO3－、H2CO3，正确；B、pH=2的醋酸中：c(H+)=0．01mol•L－1，错误；C、氯化铵溶液呈酸性，离子浓度最大的是不水解的离子Cl－，错误；D、二者的体积、浓度均相同，恰好完全反应，反应后的溶液呈碱性，根据电荷守恒，所以有c(Na+)>c(NH4+),错误，答案选A。

单项选择题

患者，男性，50岁。左耳鸣40天。初为间断性，数天后改为持续性，高调如蝉叫。近15天左面部麻木，咀嚼无力，并伴头晕，非旋转性走路不稳，左偏。2天前左耳听力下降，无耳流脓史。本病最可能的诊断是（）。

A.面神经瘤

B.梅尼埃病

C.突发性聋

D.听神经瘤

E.脑梗塞
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问答题

Many people are concerned about the economic condition of developing countries — countries whose average per capita GNP is a fraction of that in more industrialized countries. However, for the developing countries, there exist many obstacles to their economic development.
One major obstacle to economic development is population growth. (46) The populations of most developing countries grow at a rate much faster than those of industrialized countries, and the population of some of them is so large that there is barely enough fertile land and other resources to support it. Many less developed countries depend extensively on agriculture, adding to the problem. In these countries, an incentive to have many children exists. (47) Most farms are worked by families, and children can work in the fields at an early age because most farmers believe that more children means more workers. In addition, having many children ensures the parents that someone will look after them in their old age.
Another problem for developing countries is increasing average life expectancy—the average remaining lifetime for persons who reach a certain age. Better education, international aid, and emphasis on health-care facilities help people live longer. A high life expectancy coupled with a high crude birthrate makes it difficult to increase per capita GNP.
Another obstacle to economic growth is limited natural resources. No country can develop beyond its resource potential. Unproductive land or a harsh climate can limit economic growth. (48) Some countries may be fortunate enough to discover a valuable mineral to finance their economic development, but most focus on agriculture and, as a result, their economic growth is hindered by the shortage of natural resources or energy sources needed for industry.
Still another obstacle to economic development is a lack of appropriate education and technology. Any developing countries do not have a highly literate population or the high level of technical skills needed to build an industrial society. In addition, most do not have money to train engineers and scientists. (49) Many developing countries cannot even afford to provide free public education for school-age children, and in those that can, not every child is able to take advantage of it because most of them have to work to help feed their families. As a result, much of the population may not have the basic skills needed to continue with higher education when it is offered.
Government also can be an obstacle to economic development. A country whose government often changes will have a hard time developing economically. Such constant changes impede long-term planning. (50) Economic development is made even more difficult if the political changes occur through violent revolution where industrial facilities may be destroyed.
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