— ______ will the supper be ready? I’m very hungry.

— In a minute.

A．How soon

B．How long

C．How much

D．How often

已知X₁，X₂，…，X_n是来自正态总体N(0，σ²)容量为n(n＞1)的简单随机样本，样本均值与方差分别为

试求统计量

的期望

与方差