The accompanying data file contains monthly data on rent (Rent, in $) along with square footage (Sqft), number of bedrooms (Bed), and number of bathrooms (Bath) of 80 rental units. Consider two linear regression models for Rent. Model 1 uses Bed, Bath, and Sqft, whereas Model 2 also allows the interaction between Bed and Sqft. 

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.