7设设1,x2,···,x6是来自N(μ,σ^2)的样本, ^2=dfrac (1)(5)sum _(i=1)^6(({X)_(i)-overline (X))}^2, 则 ((S)^2)= () .-|||-(A) dfrac (2)(5)(sigma )^4 (B) dfrac (4)(5)(sigma )^4 (C) dfrac (2)(5)(sigma )^2 (D) dfrac (4)(5)(sigma )^2