Let {X1, X2, . . . , Xn} be a random sample from an Exponential distribution with parameter ?. This means that E(Xi) = ? and V (Xi) = ? 2 for all i = 1, 2, . . . , n. We propose three estimators for estimating ? as: T1 = X1 + X2 2 , T2 = X1 + X2 + X3 3 , T3 = X1 + X2 + X3 + X4 4 . Which one is unbiased and has the smallest variance among these three estimators? (A) T1 (B) T2 (C) T3 (D) both T1 and T2