5、设 sim N(1,9) ,X1,X2,···X9为X的样本,记 overline (X)=dfrac (1)(n)sum _(i=1)^n(X)_(i) 则有 ()-|||-A. dfrac (overline {X)-1}(3)sim N(0,1) B. overline (X)-1sim N(0,1) C. dfrac (overline {X)-1}(9)sim N(0,1) D. dfrac (overline {X)-1}(sqrt {3)}sim N(0,1)