设X1,X2,···,Xn为总体X的样本,-|||-(X=k)=dfrac (lambda )(k!)(e)^-lambda =-|||-0,1,2,···,记 overline (X)=dfrac (1)(n)sum _(i=1)^n(X)_(ij)-|||-,({S)_(n)}^2=dfrac (1)(n)sum _(i=1)^n(({X)_(i)-overline (X))}^2,-|||-则下列结论成立的是() ()-|||-A.Sn^2是λ的无偏估计-|||-B.X是λ的一致估计-|||-C.X是λ的无偏估计-|||-D. _(n)^2 是λ的一致估计