7.设X1,X2,···, _(n)(ngeqslant 2) 为来自正态总体N(μ,1)的简单随机样本,若 overline (X)=-|||-dfrac (1)(n)sum _(i=1)^n(X)_(i) ,则下列结论中不正确的是 ()-|||-A. sum _(i=1)^n(({X)_(i)-mu )}^2sim (chi )^2(n) B. (({X)_(n)-(X)_(1))}^2sim (X)^2(1)-|||-C. sum _(i=1)^n(({X)_(i)-overline (X))}^2sim (X)^2(n-1) . D. ((overline {X)-mu )}^2sim (X)^2(1)