5.设总体X的数学期望为μ,方差为σ ^2,(x1,x2)是X的一个样本,则在下-|||-述的4个估计量中, () 是最优的.-|||-A、 (hat {mu )}_(1)=dfrac (1)(5)(X)_(1)+dfrac (4)(5)(X)_(2) B、 (hat {mu )}_(2)=dfrac (1)(8)(X)_(1)+dfrac (1)(4)(X)_(2)-|||-C、 (hat {mu )}_(3)=dfrac (1)(2)(X)_(1)+dfrac (1)(2)(X)_(2) D、 (hat {mu )}_(4)=dfrac (1)(2)(X)_(1)+dfrac (1)(3)(X)_(2)