Let $X_1, X_2, X_3, X_4$ be i.i.d. observations from a distribution with mean $mu$ and variance $sigma^2$. Consider the following four estimators of $mu$: $hat{mu}_1 = X_1$, $hat{mu}_2 = (X_2 + X_3)/2$, $hat{mu}_3 = 0.1X_1 + 0.2X_2 + 0.3X_3 + 0.4X_4$, $hat{mu}_4 = ar{X}$. Show that all four estimators are unbiased.
2. Calculate the variance of each estimator. Which one has the smallest variance?