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SAT官方每日一題附答案和解析[閱讀](2019年6月14日)

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Critical Reading>Select an Answer

Plants, and all other living things, require nitrogen for growth; it is an essential component of nucleic acids and proteins. Although air is mostly nitrogen, this gaseous form is inaccessible to plants and must be fixed into ammonium to render it biologically relevant. Soil bacteria called rhizobia fix nitrogen, but to do this they must first take up residence inside the roots of legumes like pea, alfalfa, clover, and soybean.

Soon after a legume begins to grow, rhizobia invade its root hairs and multiply, causing the plant to form specialized organs—nodules—that contain the proliferating bacteria. This symbiotic arrangement benefits both parties: legumes can thrive without nitrogen fertilizers only if they have functional nitrogen-fixing nodules, while the bacteria receive the energy needed to multiply and fix nitrogen from the plant. When the plant dies, the fixed nitrogen is released into the soil so other plants can use it. This process has significant implications for agriculture, as nitrogen is the most common nutrient deficient in the earth's soil and, thus, the one most commonly supplied by chemical fertilizers.

Rhizobia are a diverse group taxonomically, genetically, and metabolically. They can be found in distant genera. Their symbiotic trait appears to have arisen independently multiple times by horizontal transfer of genes. However, it is not thought that this horizontal gene transfer is sufficient to confer symbiosis, or to explain the biodiversity of rhizobia. There must be selective pressures preventing or permitting the expression of the acquired symbiosis trait and adaptive mechanisms to deal with these pressures. But neither the pressures nor the measures taken to circumvent them are known.

The author indicates that the relationship between soil bacteria and legumes is ________

(A) mutually advantageous to both the bacteria and the plants.
(B) responsible for a decrease in the plants' survival rates.
(C) more beneficial to the bacteria than to the plants.
(D) a possible explanation for the soil's nitrogen deficiency.

重點單詞   查看全部解釋    
essential [i'senʃəl]

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n. 要素,要點
adj. 必要的,重要的,本

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sufficient [sə'fiʃənt]

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adj. 足夠的,充分的

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survival [sə'vaivəl]

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n. 生存,幸存者

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thrive [θraiv]

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vi. 興旺,繁榮,茁壯成長

 
contain [kən'tein]

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vt. 包含,容納,克制,抑制
vi. 自制

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diverse [dai'və:s]

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adj. 不同的,多種多樣的

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multiply ['mʌltiplai]

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vt. 乘,增加
vi. 擴大,繁衍,做乘法<

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beneficial [.beni'fiʃəl]

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adj. 有益的,有利的

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trait [treit]

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n. 特點,特征,特性,一筆,少量

 
bacteria [bæk'tiəriə]

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n. (復數)細菌

 
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