5.设总体X的数学期望 (X)=u, 方差 (X)=(sigma )^2, X1,X2,X3是来自总体X-|||-的简单随机样本,则下列μ的估计量中最有效的是 ()-|||-(A) dfrac (1)(4)(X)_(1)+dfrac (1)(2)(X)_(2)+dfrac (1)(4)(X)_(3) (B) dfrac (1)(3)(X)_(1)+dfrac (1)(3)(X)_(2)+dfrac (1)(3)(X)_(3)-|||-(C) dfrac (3)(5)(X)_(1)+dfrac (4)(5)(X)_(2)-dfrac (2)(5)(X)_(3) (D) dfrac (1)(6)(X)_(1)+dfrac (2)(6)(X)_(2)+dfrac (1)(2)(X)_(3)