12. Suppose a certain type of fertilizer has an expected yield per acre of , with variance o, whereas the expected yield for a second type of fertilizer is , with the same variance o. Let S and S denote the sample variances of yields based on sample sizes n, and n,, respectively, of the two fertilizers. Show that the pooled (combined) estimator (n-1)+(n-1)S n + m-2 is an unbiased estimator of o