12.设X1,X2,X3,X4是来自均值为θ的指数分布总体的样本,其中θ未知.设有估计量:-|||-._(1)=dfrac (1)(6)((X)_(1)+(X)_(2))+dfrac (1)(3)((X)_(3)+(X)_(4)) ,_(2)=dfrac (1)(5)((X)_(1)+2(X)_(2)+3(X)_(3)+4(X)_(4)) ,-|||-_(3)=dfrac (1)(4)((X)_(1)+(X)_(2)+(X)_(3)+(X)_(4)) .-|||-(1)指出T1,T2,T33中哪几个是θ的无偏估计量.(2)在上述θ的无偏估计中指出哪一个较为-|||-有效.