[单项选择]　　 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 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. 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 fina
A. elevation
B. change
C. advantage
D. condition

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