We want analyze the production function of a set of 200 firms for 2014. To that end, we dispose of a data base that contains
informatio about output (Y), number of skilled workers (L1), number of non-skilled workers (L2), R&D capital (RDK) and 
non-productive capital (NPK).
With this information, ask to the following questions:
A) estimate the production function by OLS
B) estimate this function by using the HAC correction
C) estimate this function by using GLS
D) estimate the best possible model
E) interpret the model in economci terms.