Let us consider that we want to study the sales function of a company. To this end, we dispose of a database (ex3.wf1)
that covers  the period 1970: 1-2013:4 and contains information about the following variables:
- St, sales of the company
- P1t, average price of the product
- P2t, average price of the main competitor
- P3t, average market price
- P4t, average price in the secondary market
- Rt, average income of the period for the geographical area of ??reference
- Gpt, total spending on advertising
- Ev1, estimated share of the promotion campaign #1
- Ev2, estimated share of the promotional campaign #2

with this information, you should reply to the following questions:

a) estimate the most appropriate model.

Using this model, you should test for:

b) the hypothesis of individual significance of the regressors.
c) the hypothesis of joint significance of the regressors.
d) the hypothesis of joint significance of the explanatory variables.
e) analyze the explanatory power of the model.
f) if we assume that the position parameters of the explanatory variables are the corresponding elasticities, interpret the model in economic terms.
g) predict the value of sales for the period 2014