设 sim N(1,4) ,X1,X2,···,Xn为X的样本,则 () .-|||-A. dfrac (overline {X)-1}(2)sim N(0,1) . B. dfrac (overline {X)-1}(4)sim N(0,1)-|||-C. dfrac (X-1)(2/sqrt {n)}sim N(0,1) D. dfrac (overline {X)-1}(sqrt {2)}sim N(0,1)