(a). Consider the following linear model. yi = B1 + B2xzi + . . . BKIKi + ei, where i = 1, . .., N and (X2i, . . ., "Ki) are regressors. Let's denote X - {x2i, . . ., XKi | i = 1, . .., N} for notational simplicity. What are the four key assumptions for the OLS estimator to be the Best Linear Unbiased Estimator (BLUE). (4 points) (b). Provide some brief comments on each of these assumptions. (4 points) (c). Suppose that an economic theory suggests that Varlei | X] = o?, which varies with respect to the index 2. Now, one runs the least squares regression using the data without any adjustment, what can we say about the regression results? (4 points)