We want to study the demand function of a product. To that end, we dispose of the following set of variables:
- sales (dependent variable)
- pp:  price of the product
- pp1: price of the main competitor 
- pp2: average price of the market
- GDP: Gross Domestic Product
- DI:  Disposable Income
- ICC: conficence index of the consumers.

The sample covers the period 1992:1-2016:4. With this information, you have to:
A) estimate the a linear relationship between the dependent variables and the rest of the variables
B) Check for the appropriateness of the linear funcional form. 
C) Analyze the possible existence of collinearity problems
D) Test for the possible presence of a structural break
E) improve the initial specification in such a way that the final model solves the previous problems (if exist) and can be interpreted in economic terms.