4.设随机变量X1,X2,···,Nn相互独立同分布, ({X)_(i)}^k=(mu )_(k)(i=1,2,... ,n) ,则-|||-由切比雪夫不等式,有 (|dfrac (1)(n)sum _(i=1)^n({X)_(i)}^2-(mu )_(2)|geqslant s)leqslant () .-|||-(A) dfrac ({mu )_(4)-({mu )_(2)}^2}(n{varepsilon )^2} (B) dfrac ({mu )_(1)-({mu )_(2)}^2}(sqrt {n{s)^2}} (C) dfrac ({mu )_(2)-({mu )_(1)}^2}(n{varepsilon )^2} (D) dfrac ({mu )_(2)-({mu )_(1)}^2}(sqrt {n{s)^2}}