选项中的α代表了啥

问题如下：

We use interval estimation to study the mean of populations. If the population is normally distributed but the variance is unknown, how can we construct confidence intervals?

选项：

Both The t-statist and the Z-statistic

The Z-statistic at α/2 with n - 1 degrees of freedom.

The t-statistic at α/2 with n - 1 degrees of freedom.

解释：

C is correct.

解析：（总体）方差已知用z，（总体）方差未知用t，非正态小样本不可估计。