设X1,X 2,···,"是总体X的简单随机样-|||-本, (X)=mu ,(X)=(sigma )^2 ,记-|||-.overline (X)=dfrac (1)(n)sum _(i=1)^nX , ^2=dfrac (1)(n-1)sum _(i=1)^n(({X)_(i)-overline (X))}^2-|||-则下列不正确的是 ()-|||-A. X是μ的矩估计量-|||-B. X是μ的无偏估计量-|||-C.S^2是 (sigma )^2 的矩估计量-|||-D.S^2是σ^2的无偏估计量