Recall that Figure 13.26(a) gives n = 16 weekly values of Pages' Bookstore sales (y), Pages' advertising expenditure (x1), and competitor's advertising expenditure (x2). Use MINITAB or Excel, to fit the model
y = β0 + β1x1 + β2x2 + ε
and plot the model's residuals versus time. Does the residual plot indicate that using x2 in the model has removed the autocorrelation that is apparent in Figure 13.26(b)?