设X1,X2为来自正态总体N(μ,σ^2 )的样本,下列不是μ的无偏估计量的是-|||-A (mu )_(1)=dfrac (2)(3)(X)_(1)+dfrac (1)(3)(X)_(2)-|||-)A (mu )_(2)=dfrac (2)(3)(X)_(1)+dfrac (1)(2)(X)_(2)-|||-C (mu )_(3)=dfrac (3)(4)(X)_(1)+dfrac (1)(4)(X)_(2)-|||-D (mu )_(4)=dfrac (4)(5)(X)_(1)+dfrac (1)(5)(X)_(2)