Let a represent the amount of nitrogen fertilizer applied to a cornfield, in pounds per acre. Let y ( ) represent the yield of the cornfield, in bushels of corn per acre. The function r(a) = y'(a), measured in bushels per pound, represents the rate at which yield increases per pound of additional nitrogen fertilizer applied, at an application level of a pounds per acre. Choose the best interpretation of the expression S50 r(x) dx. The yield of the cornfield, in bushels per acre, 100 days after fertilizer was first applied. The increase in the yield of the cornfield between the 50th and 100th day after fertilizer was first applied. The change in the yield of the cornfield, in bushels per acre, when the amount of nitrogen fertilizer applied is increased from 50 pounds per acre to 100 pounds per acre. O The yield of the cornfield, in bushels per acre, when 100 pounds of nitrogen fertilizer are applied. The rate of increase of the yield of the cornfield 100 days after fertilizer was first applied. The change in the rate of increase in the yield of the cornfield between the 50th and 100th day after fertilizer was first applied. None of the others The rate of increase of the yield of the cornfield when 100 pounds of nitrogen fertilizer are applied