A.≤1/4
B.≤8/9
C.≤80/81
D.≥8/9
[单选题]A.≤1/4B.≤8/9C.≤80/81D.≥8/9
[单选题]A.B.C.D.
[单选题]A.S是σ的无偏估计B.C.D.
[单选题]A.线性相关B.线性无关C.正交D.平行
[单选题]A.1B.2C.3D.4