设都是参数θ的无偏估计量，且，，，则下列结论正确的是(    )
   A．有效    B．有效    C．有效    D．不能确定

A.174.24.96:6088/Latex/latex.action?latex=XGhhdHtcdGhldGF9X3sxfe%2B8jFxoYXR7XHRoZXRhfV97Mn3vvIxcaGF0e1x0aGV0YX1fezN9'
B.174.24.96:6088/Latex/latex.action?latex=RChcaGF0e1x0aGV0YX1fezF9KT0wLjE%3D'
C.174.24.96:6088/Latex/latex.action?latex=RChcaGF0e1x0aGV0YX1fezJ9KT0wLjQ%3D'
D.174.24.96:6088/Latex/latex.action?latex=RChcaGF0e1x0aGV0YX1fezN9KT0wLjI%3D'
E.F.174.24.96:6088/Latex/latex.action?latex=XGhhdHtcdGhldGF9X3szfeavlFxoYXR7XHRoZXRhfV97MX0%3D'
G.H.174.24.96:6088/Latex/latex.action?latex=XGhhdHtcdGhldGF9X3syfeavlFxoYXR7XHRoZXRhfV97MX0%3D'
I.J.174.24.96:6088/Latex/latex.action?latex=XGhhdHtcdGhldGF9X3sxfeavlFxoYXR7XHRoZXRhfV97Mn0%3D'