11.15 Refining the GPA model using all six explanatory variables: Residual checks. Figure 11.11 (page 631) provides a list of the top models based on R². Let’s look more closely at the four models listed with p = 3 and p = 4. Fit each of these models to the data and obtain the residuals. Do the data, at least approximately, meet the conditions of the multiple regression model? Provide some plots to support your opinion.