Suppose a certain type of fertilizer has an expected yield per acre of μ1 with variance s2, whereas the expected yield for a second type of fertilizer is m2 with the same variance s2. Let S21 and S22 denote the sample variances of yields based on sample sizes n1 and n2, respectively, of the two fertilizers. Show that the pooled (combined) estimator μ1 with variance σ2, is an unbiased estimator of σ2.