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GRE阅读模拟在线练习附答案一百四十

2018-09-03 14:07:00来源：网络

　　GRE阅读真题是大家复习备考中必备的资料，利用真题进行模拟考试的练习，可以帮助我们发现备考中的不足，重点来复习自己的短板，从而提升GRE阅读的速度，提高我们的分数。

点击查看》》》【GRE阅读真题在线练习附答案汇总】

　　Two relatively recent independent developments stand behind the current major research effort on nitrogen fixation, the process by which bacteria symbiotically render leguminous plants independent of nitrogen fertilizer. The one development has been the rapid, sustained increase in the price of nitrogen fertilizer. The other development has been the rapid growth of knowledge of and technical sophistication in genetic engineering. Fertilizer prices, largely tied to the price of natural gas, huge amounts of which go into the manufacture of fertilizer, will continue to represent an enormous and escalating economic burden on modern agriculture, spurring the search for alternatives to synthetic fertilizers. And genetic engineering is just the sort of fundamental breakthrough that opens up prospects of wholly novel alternatives. One such novel idea is that of inserting into the chromosomes of plants discrete genes that are not a part of the plants’ natural constitution: specifically, the idea of inserting into nonleguminous plants the genes, if they can be identified and isolated, that fit the leguminous plants to be hosts for nitrogen-fixing bacteria. Hence, the intensified research on legumes.

　　Nitrogen fixation is a process in which certain bacteria use atmospheric nitrogen gas, which green plants cannot directly utilize, to produce ammonia, a nitrogen compound plants can use. It is one of nature’s great ironies that the availability of nitrogen in the soil frequently sets an upper limit on plant growth even though the plants’ leaves are bathed in a sea of nitrogen gas. The leguminous plants—among them crop plants such as soybeans, peas, alfalfa, and clover—have solved the nitrogen supply problem by entering into a symbiotic relationship with the bacterial genus Rhizobium; as a matter of fact, there is a specific strain of Rhizobium for each species of legume. The host plant supplies the bacteria with food and a protected habitat and receives surplus ammonia in exchange. Hence, legumes can thrive in nitrogen-depleted soil.

　　Unfortunately, most of the major food crops—including maize, wheat, rice, and potatoes—cannot. On the contrary, many of the high-yielding hybrid varieties of these food crops bred during the Green Revolution of the 1960’s were selected specifically to give high yields in response to generous applications of nitrogen fertilizer. This poses an additional, formidable challenge to plant geneticists: they must work on enhancing fixation within the existing symbioses. Unless they succeed, the yield gains of the Green Revolution will be largely lost even if the genes in legumes that equip those plants to enter into a symbiosis with nitrogen fixers are identified and isolated, and even if the transfer of those gene complexes, once they are found, becomes possible. The overall task looks forbidding, but the stakes are too high not to undertake it.

　　The primary purpose of the passage is to

　　A.expose the fragile nature of the foundations on which the high yields of modern agriculture rest

　　B.argue that genetic engineering promises to lead to even higher yields than are achievable with synthetic fertilizers

　　C.argue that the capacity for nitrogen- fixing symbioses is transferable to nonleguminous plants

　　D.explain the reasons for and the objectives of current research on nitrogen-fixing symbioses

　　E.describe the nature of the genes that regulated they symbiosis between legumes and certain bacteria

　　According to the passage, there is currently no strain of Rhizobium that can enter into a symbiosis with

　　A.alfalfa

　　B.clover

　　C.maize

　　D.peas

　　E.soybeans

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