3.设总体 approx N(mu ,(sigma )^2), X1,X2···Xn是来自该总体的简单随机样本,则-|||-dfrac (1)({sigma )^2}sum _(i=1)^n(({X)_(i)-mu )}^2 是: ()-|||-(A) (n-1); (B)t(n); C) ^2(n-1); (D)x^2(n).