【单选题】

The wide variety of climates in NorthAmerica has helped spawn a complex pattern of　　 soil regions. In general, the realm’’s soils also reflect the broad environmental partitioning　　 into "humidAmerica" and "aridAmeric

A、" 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 ale 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.AridAmerica’’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 theCentral 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 Mississippi Valley.　 　Natural vegetation patterns could be displayed on a map of NorthAmerica, but the　 　enormous human modification of the NorthAmerican environment in modem times has　 　all but reduced this regionalization scheme to the level of the hypothetical. Nonetheless,(20) the humidAmerica-aridAmerica 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 NorthAmerica tent to make　 　a broad transition by latitude. In theCanadian 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 StatesAs one proceeds toward the Southeast, broadleaf vegetation　 　becomes dominant.AridAmerica mostly consists of short-grass prairies or　 　stepper. The only areas of true desert are in the Southwest. The word "transition" in line 23 is closest in meaning to
A、elevation
B．change
C．advantage
D．condition