5、随机变量X1,X 2,L,Xn独立且都服从N(2,4)分布,则 overline (X)=dfrac (1)(n)sum _(i=1)^n(X)_(i) 服从-|||-(A)N(0,1); (B)N(2,4n);-|||-(C)N(2n,4n); (D)N(2,4、