Among All Unbiased Estimators That Are Weighted Averages of Y₁ $\bar { Y }$

Among all unbiased estimators that are weighted averages of Y₁,..., Y_n $\bar { Y }$ , is

A) the only consistent estimator of µ_Y.
B) the most efficient estimator of µ_Y.
C) a number which, by definition, cannot have a variance.
D) the most unbiased estimator of µ_Y.