2.设总体X的期望E(X)和方差D(X)都存在,x1,x2,X3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期望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)_(3) 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)