单项选择题
A.δ4tx ,50σ2B.δ4tx ,60σ2C.δ4tx ,70σ2D.δ4tx ,80σ2E.δ4tx ,90σ2
A.3/4B.3/8C.3/64D.3/128E.3/256
A.1B.2C.3D.4E.5