The accompanying data file contains survey data for 80 participants with information on average daily time spent on mobile devices (Usage, in minutes), household income (Income, in $1,000s), lived in a rural area (Rural =1 if rural, 0 otherwise), and had a college degree (College = 1 if college graduate, 0 otherwise). Consider two predictive models for mobile device usage where Model 1 is based on Income, Rural, and College and Model 2 also includes the interaction between Rural and College. 

a. Use the holdout method to compare the predictability of the models using the first 60 observations for training and the remaining 20 observations for validation. 

b. Use the k-fold cross-validation method to compare the predictability of the models using k = 4.