Create a regression tree using the accompanying data set (predictor variables: x₁ to x₄; target: y). Select the best-pruned tree for scoring and display the full-grown, best-pruned, and minimum error trees. 

a. What is the minimum validation MSE in the prune log? How many decision nodes are associated with the minimum error? 

b. Display the best-pruned tree. How many leaf nodes are in the best-pruned and minimum error trees? 

c. What are the predictor variable and split value for the first split (root node) of the best-pruned tree? What are the rules that can be derived from the root node? 

d. What are the RMSE and MAD of the best-pruned tree?