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OFFICIAL36 Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in summary because the express ideas that are not presented in the passage or are minor ideas in the passage. The Question is worth 2 points. Plants and animals play an important role in the development of soils. Drag your answer choices to the space where they belong. To remove an answer choice, double click on it.

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Soil Formation
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Living organisms play an essential role in soil formation. The numerous plants and animals living in the soil release minerals from the parent material from which soil is formed, supply organic matter, aid in the translocation (movement) and aeration of the soil, and help protect the soil from erosion. The types of organisms growing or living in the soil greatly influence the soil's physical and chemical characteristics. In fact, for mature soils in many parts of the world, the predominant type of natural vegetation is considered the most important direct influence on soil characteristics. For this reason, a soil scientist can tell a great deal about the attributes of the soil in any given area simply from knowing what kind of flora the soil supports. Thus prairies and tundra regions, which have characteristic vegetations, also have characteristic soils.

The quantity and total weight of soil flora generally exceed that of soil fauna. By far the most numerous and smallest of the plants living in soil are bacteria. Under favorable conditions, a million or more of these tiny, single-celled plants can inhabit each cubic centimeter of soil. It is the bacteria, more than any other organisms, that enable rock or other parent material to undergo the gradual transformation to soil. Some bacteria produce organic acids that directly attack parent material, breaking it down and releasing plant nutrients. Others decompose organic litter (debris) to form humus (nutrient-rich organic matter). A third group of bacteria inhabits the root systems of plants called legumes. These include many important agricultural crops, such as alfalfa, clover, soybeans, peas, and peanuts. The bacteria that legumes host within their root nodules (small swellings on the root) change nitrogen gas from the atmosphere into nitrogen compounds that plants are able to metabolize, a process, known as nitrogen fixation, that makes the soil more fertile. Other microscopic plants also are important in soil development. For example, in highly acidic soils where few bacteria can survive, fungi frequently become the chief decomposers of organic matter.

More complex forms of vegetation play several vital roles with respect to the soil. Trees, grass, and other large plants supply the bulk of the soil's humus. The minerals released as these plants decompose on the surface constitute an important nutrient source for succeeding generations of plants as well as for other soil organisms. In addition, trees can extend their roots deep within the soil and bring up nutrients from far below the surface. These nutrients eventually enrich the surface soil when the tree drops its leaves or when it dies and decomposes. Finally, trees perform the vital function of slowing water runoff and holding the soil in place with their root systems, thus combating erosion. The increased erosion that often accompanies agricultural use of sloping land is principally caused by the removal of its protective cover of natural vegetation.

Animals also influence soil composition. The faunal counterparts of bacteria are protozoa. These single-celled organisms are the most numerous representatives of the animal kingdom, and, like bacteria, a million or more can sometimes inhabit each cubic centimeter of soil. Protozoa feed on organic matter and hasten its decomposition. Among other soil-dwelling animals, the earthworm is probably the most important. Under exceptionally favorable conditions, up to a million earthworms (with a total body weight exceeding 450 kilograms) may inhabit an acre of soil. Earthworms ingest large quantities of soil, chemically alter it, and excrete it as organic matter called casts. The casts form a high-quality natural fertilizer. In addition, earthworms mix of soil both vertically and horizontally, improving aeration and drainage.

Insects such as ants and termites also can be exceedingly numerous under favorable climatic and soil conditions. In addition, mammals such as moles, field mice, gophers, and prairie dogs sometimes are present in sufficient numbers to have significant impact on the soil. These animals primarily work the soil mechanically. As a result, the soil is aerated broken up, fertilized, and brought to the surface, hastening soil development.

14.Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in summary because the express ideas that are not presented in the passage or are minor ideas in the passage. The Question is worth 2 points. Plants and animals play an important role in the development of soils. Drag your answer choices to the space where they belong. To remove an answer choice, double click on it.

A.Small organisms such as bacteria and fungi create and enrich soils by breaking down rocks and organic matter.

B.Such agricultural crops as alfalfa, clover, soybeans, peas, and peanuts provide nutrient minerals to the soil as they die and decompose.

C.Trees and other large plants aid soil development by providing natural fertilizer as they decompose, and by protecting the soil against erosion.

D.Prairie and tundra soils can be found in areas where conditions are generally not favorable for soil development.

E.Certain animals block soil development by digging holes in the ground that prevent nutrients from reaching some areas of the soil.

F.Protozoa, earthworms, and small mammals contribute to soil development by aerating, fertilizing, and mixing the soil.

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正确答案:ACF
题目解析:
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本题选项A说细菌和真菌通过分解石头和有机物来产生土壤,概括了第二段的主要内容,可以选;选项B说农作物死掉分解以后可以给土壤提供营养矿物,是对的,但是只是第二段的一个细节,不选;选项C说树木和别的大植物既可以分解以后给土壤提供营养,也可以帮助固定水土防止流失,对的,对应概括了第三段的主要内容,可以选;选项D说草原和冻原是出现在土壤不容易发展的地方的,这个原文里并没有明说;选项E说一些动物会挖洞,阻碍土壤里营养的传输,这个原文没提,不选;选项F说一些小的动物可以通过充气,施肥和搅拌等等办法来促进土壤生成,概括了第五段的内容,可以选。最后选择A,C,F。

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