Information is given below for each student on two variables not included in the model, namely, intelligence test score (X2) and high school class rank percentile (X3). (Note that larger class rank percentiles indicate higher standing in the class, e.g., 1% is near the bottom of the class and 99% is near the top of the class.) Plot the residuals against X2 and X3 on separate graphs to ascertain whether the model can be improved by including either of these variables. What do you conclude