The accompanying data file lists monthly data on vehicle miles traveled in the U.S. (in millions). For cross-validation, let the training and the validation sets comprise the periods from Jan-12 to Dec-16 and Jan-17 to Sep-18, respectively. 

a. Use the training set to estimate the linear and the exponential trend models with seasonal dummy variables and compute the resulting MSE, MAD, and MAPE for the validation set. 

b. Determine the preferred model and reestimate it with the entire data set to forecast vehicle miles for October 2018.