4. Consider the following quarterly sales data for a local coffee shop. Year/Quarter Coffee sales Year/Quarter Coffee sales Year 1 - Quarter 1 48 Year 2 - Quarter 3 18 Year 1 - Quarter 2 32 Year 2 - Quarter 4 36 Year 1 - Quarter 3 19 Year 3 - Quarter 1 40 Year 1 - Quarter 4 34 Year 3 - Quarter 2 28 Year 2 - Quarter 1 45 Year 3 - Quarter 3 22 Year 2 - Quarter 2 34 Year 3 - Quarter 4 38 (a) Let Year 1 and Year 2 be the training period and Year 3 the validation period. Find the seasonal naive forecasts for each quarter of Year 3. (b) Based on your forecasts in part (a), calculate the errors for the validation period. (c) Calculate the RMSE (root mean squared error) for the validation period. (d) Assuming you are content with the forecasting or predictive accuracy of this model, find the seasonal naive forecasts for each quarter of Year 4