We want to study the investment function of a particular economy. To that end, we dispose of information about
Investment (INV), GDP, unemployment rate (ur), short-run real interest rate (srir) and long-run real interest rate (lrir).
The sample covers the 1976:1-2000:4 quarterly period.
Using this information:

A) estimate the model by OLS
B) estimate the model by using HAC corrections.
C) estimate the model by using FGLS.
D) estimate the best possible model. 
E) interpret the final model in economic terms.