The accompanying data file contains monthly data on housing starts (in 1,000s) in the U.S. For cross-validation, let the training and the validation sets comprise the period from Jan-11 to Dec-16 and Jan-17 to Nov-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 housing starts for December of 2018.