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One of the primary shortcomings of the HomesteadAct of 1862, a program through which the federalgovernment offered free plots of land to US citizenswilling to establish farms in the American West, wasthat the plots were too small to sustain a farm.Plots in the Great Plains were best left halfunplanted each growing season to conserve water. The Great Plains region receivedtwenty or fewer inches of rain per year and was considered semiarid. With farmsconsisting of a mere 160 acres, however, this practice would not have produced sufficientharvests. Moreover, raising cattle was impossible because the tiny homesteads would quicklybecome overgrazed. According to geologist and explorer John Wesley Powell, sustainablehomesteads on the Great Plains actually needed to be a staggering 2,560 acres.
Which choice most effectively combines the underlined sentences?
A.Receiving twenty or fewer inches of rain per year, land in the Great Plains was best left halfunplanted each growing season; the reason for this was to conserve water in this semiaridregion.
B.To conserve water, land in the semiarid Great Plains region was best left half unplanted eachgrowing season; consequently, this region received twenty or fewer inches of rain per year.
C.Land in the Great Plains region, being semiarid, was best left half unplanted each growingseason to conserve water, and it received twenty or fewer inches of rain per year.
D.Land in the semiarid Great Plains region received twenty or fewer inches of rain per year, soplots there were best left half unplanted each growing season to conserve water.
重点单词讲解：
sufficient [sə'fiʃənt] adj. 足够的，充分的
primary ['praiməri] adj. 主要的，初期的，根本的，初等教育的
effectively [i'fektivli] adv. 事实上，有效地
logical ['lɔdʒikəl] adj. 符合逻辑的，逻辑上的，有推理能力的
willing ['wiliŋ] adj. 愿意的，心甘情愿的
conserve ['kɔnsə:v,kən'sə:v] n. 蜜饯，果酱
efficient [i'fiʃənt] adj. 效率高的，胜任的
conjunction [kən'dʒʌŋkʃən] n. 连词，结合，关联，(事件等的)同时发生
consequently ['kɔnsikwəntli] adv. 所以，因此
sustain [səs'tein] vt. 承受，支持，经受，维持，认可
答案：D
解析：
Choice D is the best answer because it mosteffectively combines the sentences in a logical andefficient way. The coordinating conjunction "so"indicates the cause-effect relationship between thesentences.