问题如下图：
选项：
A. 
B. 
C. 
提问：请问这题为啥不选a呢？谢谢老师 解释：
NO.PZ2018062016000137 问题如下 Assume ththe varianis unknown, whiof the following statements on a hypothesis tests of the population meis most accurate? A.If the sample is small, it's a t-test. B.If the sample is large, it's a t-test. C.If the sample is small, it's a z-test. B is correct. If the population samplehunknown variance, the sample is large, or the sample is small but the population sampleis normally strubute or approximately normally stribute then the test statistic for hypothesis tests concerning a single population mean. 老师，我想请问下这道题A和B如何bianbie1正确的答案，因为方差未知可以排除C，但是A，B感觉都没有明显的问题，反而是A更说得过去一点。谢谢
方差未知，小样本用T分布，大样本中Z分布，不对吗
本题和样本数量超过30这个概念有关吗
按照老师上课讲的，a为什么不对
