资料分析：小麦、水稻、棉花的分类地位如下：门种子植物门种子_爱下问搜题

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问题实验题

资料分析：小麦、水稻、棉花的分类地位如下：

门种子植物门种子植物门种子植物门

纲单子叶植物纲单子叶植物纲双子叶植物纲

目禾本目禾本目锦葵目

科禾本科禾本科锦葵科

属小麦属稻属棉属

种小麦水稻棉花

（1）你会发现，小麦与水稻之间共同特征较___________ ，原因是___________，小麦与棉花之间亲缘关系较___________，原因是它们同门不同纲。

（2）下列有关种的叙述中，错误的是（）

A．同种生物必同属

B．同种的哺乳动物，可以通过有性生殖产生后代，后代也具有生殖能力

C．不同种的植物，可以通过人工授粉产生后代

D．同属必同种

答案

（1）多小麦和水稻同属禾本科植物远（2）D

题目分析：（1）生物分类单位中，分类单位越小，生物间相似特征越多，亲缘关系最近，分类单位越大，生物间的亲缘关系越远，小麦与水稻同科，所以二者间的共同特征较多，小麦与棉花同门不同纲，亲缘关系比小麦与水稻间的远些。（2）生物分类是同种必同属，但是同属不一定同种。

单项选择题

血/气分布系数最低的麻醉药是（）。

A.七氟烷

B.氟烷

C.恩氟烷

D.异氟烷

E.地氟烷

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.