4.设X1,X2,X3是来自总体X的样本,下列关于总体均值的估计中,-|||-a. ((X)_(1)+(X)_(2)+(X)_(3))/3 b.X1-|||-c. ((X)_(1)+(X)_(2))/2 d. (2(X)_(1)+(X)_(2)+(X)_(3))/4-|||-则无偏估计是 __ ,其中最有效的是 __