Consider the antidiabetic drug sales time series which can be loaded using the following code snippet. diabetes a. Fit the following exponential smoothing models on the entire time series: Holt-Winters with multiplicative noise and seasonality, Holt-Winters with a log transformation, with additive noise and seasonality, Holt-Winters with multiplicative noise and seasonality, and damping. b. Make ACF plots for the innovation residuals of these three models. What can you say about stationarity of the residuals from the plot? c. Calculate the p-value from a Ljung-Box test on the residuals with lag h = 8. What can you say about the stationarity of the residuals from the p-value? What does this mean about the model? d. Perform time series cross-validation for the three methods, using init = 50 and .step 10, and with the forecast horizon h 4. Which method has the best RMSSE? How = = many data points is the error averaged over in total?