2.设总体X的期望E(X)和方差D(X)都存在,x1,x2,x1是取自总体X的一个-|||-样本。请从下面的统计量中。找出总体X期里的最有效的无偏估计量。-|||-()-|||-A. dfrac (1)(2)(x)_(1)+dfrac (1)(3)(x)_(2)+dfrac (1)(6)(x)_(8) B. dfrac (1)(3)(x)_(1)+dfrac (1)(3)(x)_(2)+dfrac (1)(3)(x)_(3)-|||-c dfrac (1)(3)(X)_(1)+dfrac (1)(4)(X)_(2)+dfrac (1)(12)(X)_(3) D dfrac (1)(5)(x)_(1)+dfrac (1)(5)(x)_(2)+dfrac (3)(5)(x)_(3)