Questions 18 and 19 are based on the following information. A random variable X has mean μ and variance o². A random sample of 2 observations is taken from the probability distribution of X, resulting in sample random variables X₁ and X2. A researcher proposes the following four estimators of μ: X = X₁+X₂ W=X₁ 2 G=0.3X+0.7X2 Z=0.64X+0.35X2 18. Which of these estimators is an unbiased estimator of μ? a. W only b. Z only c. X and G d. X, W and G e. None, all are biased. 19. Of the unbiased estimators, the most efficient is: a. X b. W c. Z d. G e. None of the above, they all have equal variance.