The file S12_57.xlsx consists of the monthly retail sales levels of U.S. gasoline service stations.
a. Is there a seasonal pattern in these data? If so, how do you explain this seasonal pattern? Also, if necessary, de-seasonalize these data using the ratio-to-moving-average method.
b. Forecast this time series for the first four months of the next year using the most appropriate method for these data. Defend your choice of forecasting method.