Only when he had done it ______ that he had made a mistake.

A．he then realized

B．did he realize

C．before

D．he realized

设X₁，X₂，…，X_n+1是取自正态总体N(0，σ²)的简单随机样本，记

，1≤k≤n，则

A．

B．

C．kσ².
D．σ².