21 Suppose that four randomly chosen plots were treated with various levels of fertilizer, resulting in the following yields of corn: (a) Calculate the regression line of yield on fertilizer. (b) Plot the four points and the regression line. Check that the line fits the data reasonably well. (c) Explain what the intercepts a and a0 represent. (d) Estimate how much the yield is increased for every pound of fertilizer applied-that is, what is the marginal physical product of fertilizer? (e) If the value of the crop is $2 per bushel, estimate how much revenue is increased for every pound of fertilizer applied-that is, what is the marginal revenue product of fertilizer? If fertilizer costs \$.10 per pound, would it be economical to apply