Let us assume that we want to study the petrol demand function of a particular economy. To that end, the ej3.wf1 includes 
information covering the 1976:1-2000:4 period for the following variables:

- petrol sales
- income
- petrol price
- diesel fuel price)
- Promotion expenditure in sustainable consumption.

With this information

1. determine the best functional form
2. Analyze the possible presence of collinearity problems, once you have decided the most appropriate functional form.
3. Select the best possible petrol demand function
4. Interpret this model in economic terms.