2、(易)设总体 sim N((mu ,{sigma )^2}) ,其中μ未知,x1,x22,x3,x4为来自总体x的一个-|||-样本,则下列不是无偏估计的是 ()-|||-A. dfrac (1)(4)((x)_(1)+(x)_(2)+(x)_(3)+(x)_(4)) B. dfrac (1)(5)((X)_(1)+(X)_(2)+(X)_(3)+(X)_(4))-|||-C. (x)_(1)-(x)_(2) D. _(1)-2(x)_(2)+(x)_(3)+(x)_(4)