1.-|||-设X1,X2,X3是来自总体 sim N((M)_(2)(O)^2) 的一个样本,下面给出的四个统计量都是总-|||-体均值μ的无偏估计量,则它们中最有效的统计量为-|||-()。-|||-A hat (mu )_(1)=(X)_(1)-|||-B (mu )_(2)=dfrac (1)(2)(X)_(1)+dfrac (1)(3)(X)_(2)+dfrac (1)(6)(X)_(3)-|||-C (hat {mu )}_(3)=overline (X)=dfrac (1)(3)sum _(i=1)^3(X)_(3)-|||-D (mu )_(4)=dfrac (2)(5)(X)_(1)+dfrac (3)(5)(X)_(3)