4.设x1,x2,···,xn是一组样本观测值,则其标准差是 () .-|||-A、 dfrac (1)(n-1)sqrt (sum _{i=1)^n(({x)_(i)-overline (x))}^2} B、 sqrt (dfrac {1)(n-1)sum _(i=1)^n(({x)_(i)-overline (x))}^2}