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  3. On viewing the lush plant growth of a tropical rain forest, most people would conclude that the soil beneath it is rich in nutri

On viewing the lush plant growth of a tropical rain forest, most people would conclude that the soil beneath it is rich in nutri

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问题     On viewing the lush plant growth of a tropical rain forest, most people would conclude that the soil beneath it is rich in nutrients. However, although rain forest soils are highly variable, they have in common the fact that abundant rainfall washes mineral nutrients out of them and into streams. This process is known as leaching. Because of rain leaching, most tropical rain forest soils have low to very low mineral nutrient content, in dramatic contrast to mineral-rich grassland soils. Tropical forest soils also often contain particular types of clay that, unlike the mineral-binding clay of temperate forest soils, do not bind mineral ions well. Aluminum is the dominant cation (positively charged ion) present in tropical soils; but plants do not require this element, and it is moderately toxic to a wide range of plants. Aluminum also reduces the availability of phosphorus, an element in high demand by plants.
选项 A、The water in the streams flowing through tropical rain forests generally has relatively low mineral content.
B、The range of plants that grow in tropical rain forests is so wide that almost every kind of mineral in the soil gets used up.
C、The clay contained in tropical rain forest soils do not bind mineral ions well.
D、Abundant rainfall causes minerals to leach out of the soil.
答案C,D
解析 本题问的是为什么大多数热带雨林土壤的矿物质养分含量相对较低，要求选出两个答案。第1句提出了一个谬误，即人们普遍认为热带雨林土壤富含营养。而第2句则用However来引出这种说法错误的第一个原因：abundant rainfall washes mineral nutrients out of them and into streams“充足的降雨导致矿物质养分从土壤中渗出并流入河流”。其中，washes mineral nutrients out of them对应D项的causes minerals to leach out of the soil。第3、4句解释何为leaching及其对土壤的作用，是对第2句的补充。第5句给出了热带雨林土壤矿物质养分含量相对较低的另一个原因：Tropical forest soils also often contain particular types of clay that…do not bind mineral ions well“热带森林土壤也通常含有特定类型的黏土，这些黏土不能很好地结合矿物离子”，C项基本复现了原句原词。因此，本题答案为C、D两项。
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