6.设x1,x2,F,xn是来自正态总体N(μ,σ^2)的样本,X,S ^2分别是样本均值和样-|||-本方差,则下面正确的是 ()-|||-《概率统计》A2卷共5页第1页-|||-(A) overline (X)approx N(mu ,(sigma )^2) B) dfrac (sqrt {n)(sqrt (x)-mu )}(s)approx t(n)-|||-(C) dfrac (1)({sigma )^2}sum _(i=1)^n(({X)_(i)-mu )}^2approx (chi )^2(n) (D) dfrac (1)({sigma )^2}sum _(i=1)^n(({X)_(i)-overline (X))}^2approx (chi )^2(n)