4.设总体 sim N(mu ,(sigma )^2) ,(X1,X2,···,Xn)为来自总体X的一个样本,X为样本均-|||-值,则 ()-|||-(A) dfrac (1)({sigma )^2}sum _(i=1)^n(({X)_(i)-mu )}^2sim (chi )^2(n-1) (B) dfrac (n-1)({sigma )^2}sum _(i=1)^n(({X)_(i)-mu )}^2sim (chi )^2(n-1)-|||-(C) dfrac (1)({sigma )^2}sum _(i=1)^n(({X)_(i)-overline (X))}^2sim (chi )^2(n-1) (D) dfrac (n-1)({sigma )^2}sum _(i=1)^n(({X)_(i)-overline (X))}^2sim (chi )^2(n-1)