5. Let X1,X2,X3,X4 be iid rvs with ]E(X-) = 11 and var(X,-) = 0'2 > O, for 1' = 1,2,3,4. Consider the following four estimators of I11: 1 1 1 T1: 30% X2)+E(X3+X4) T2 = 6(X1+2X2+3X3+4X4) 1 1 1 T3=E(X1X2+X3+X4) T4=(X1+X2+X3)+ZX3 (a) Which of these estimators are unbiased? Show your working. (b) Among the unbiased estimators, which one has the smallest variance