8、设X1,X2,···Xn是来自总体X的样本, overline (X)=dfrac (1)(n)sum _(i=1)^n(X)_(i) 为其样本均值,则方差-|||-D(X)与D(X )的关系是 【 】-|||-A、 (overline (X))=D(X) B、 (overline (X))=dfrac (1)(n)D(X) C、 (overline (X))=noverline (X)(X) D、无法确定