The least squares (LS) estimator is the main statistical tool used in applied econometrics. It is a powerful procedure for describing and measuring the relationship among economic variables. a. What is the basic assumption (not Gauss-Markov) that allows the application of regression models? b. The least squares estimator is only applicable in linear regression models. What does linear mean and how restrictive is this assumption? c. What assumptions are applied to the residual in the linear regression model and what is the purpose of each one? d. Under GaussMarkov ordinary LS (OLS) is a BLU estimator. Describe two cases were OLS fails to be BLUE and why