求助0308 阅读26

      The wide variety of climates in North America has helped spawn a complex pattern of

    soil regions. In general, the realm’s soils also reflect the broad environmental partitioning

    into “humid America” and “arid America.” Where annual precipitation exceeds 20 inches

(50 centimeters),soils in humid areas tend to be acidic in chemical content, Since crops

(5 ) do best in soils that are neither acidic(higher in acid content) nor alkaline(higher in salt content).fertilization is necessary to achieve the desired level of neutrality between the

    two. Arid America’s soils are typically alkaline and must be fertilized back toward

    neutrality by adding acidic compounds. Although many of these dryland soils, particularly

    in the Great Plains, are quite fertile, European settlers learned over a century ago that

(10) water is the main missing ingredient in achieving their agricultural potential. In the

1970’s, certain irrigation methods were perfected and finally provided a real opportunity

    to expand more intensive farming west from the Central Lowland into the drier portions

    of the Great Plains. Glaciation also enhanced the rich legacy of fertile soils in the central

    United States,both from the deposition of mineral-rich glacial debris left by meltwater

(15) and from thick layers of fine wind-blown glacial material, called loess, in and around the

    middle laceName w:st="on">MississippilaceName> laceType w:st="on">ValleylaceType>.

       Natural vegetation patterns could be displayed on a map of North America, but the

    enormous human modification of the North American environment in modern times has

    all but reduced this regionalization scheme to the level of the hypothetical. Nonetheless,

(20) the humid America-arid America dichotomy is still a valid generalization: the natural

vegetation of areas receiving more than 20 inches of water yearly is forest, whereas the

    drier climates give rise to a grassland cover. The forests of North America tent to make

    a broad transition by latitude. In the Canadian North, needle-leaf forests dominate, but

these coniferous trees become mixed with broadleaf deciduous trees as one crosses the

(25) border into the Northeast United States. As one proceeds toward the Southeast, broadleaf vegetation becomes dominant. Arid America mostly consists of short-grass prairies or

    stepper. The only areas of true desert are in the Southwest.

26 How did glacial meltdown affect the soil in North America?

(A) It redistributed the soil types

(B)  It added salt to the soil

(C) It made the soil more neutral in content

(D) It added minerals to the soil

我感觉c正确呀,为什么答案是d呀?  我觉得glaciation不光带来了depotion of minerals 还带来了loses 所以c更全面呀?

希望大侠帮助我翻译一下红色字体的部分