We want analyze the sales of a product in different cities of a country. To that end, we dispose information for 100 cities of the following
variables:
sales: sales of the product (per capita)
p:  average price of the product.
pp1: average price of a local competitor.
gdp: Gross domestic product of the city (per capita).
di: disposable income of the city (per capita)
gw: number of shopping centers in the city. 

With this information, ask to the following questions:
A) estimate the best model by OLS
B) analyze the possible existence of approximate collinearity
C) if necessary, reestimate the model
d) interpret the final model in economic terms