5. Suppose that the random variables Y1, ..., Yn follows the following model, Yi = Bxit ci, i = 1, 2, ..., n, where x1, $2, . .., n are fixed constants, and 61, 62, . . ., En are iid N(0, o?) , and o2 is unknown. (a) Show that B = ZiziYi is an unbiased estimator of 3. (b) Find the MLE of B. (c) Calculate the exact variance of 3 and compare it to the variance of the MLE