The agronomist referred to in Exercise believed that the amount of rainfall as well as the amount of fertilizer used would affect the crop yield. She redid the experiment in the following way. Thirty greenhouses were rented. In each, the amount of fertilizer and the amount of water were varied. At the end of the growing season, the amount of corn was recorded.

a. Determine the sample regression line and interpret the coefficients.

b. Do these data allow us to infer that there is a linear relationship between the amount of fertilizer and the crop yield?

c. Do these data allow us to infer that there is a linear relationship between the amount of water and the crop yield?

d. What can you say about the fit of the multiple regression model?

e. Is it reasonable to believe that the error variable is normally distributed with constant variance?

f. Predict the crop yield when 100 kilograms of fertilizer and 1,000 liters of water are applied. Use a confidence level of 95%.