请问利用样本数据估计总体平均值的方法必要前提为“大点的样本”，但样本要多大好呢？界限在哪？[s:16]
The normal distribution is used to test about a population mean for large samples if the population standard deviation is known. "Large" is usually defined as __________ .
Select one:
a. at least 30
c. at least 12
d. at least 5% of the population sizee.
请问利用样本数据估计总体平均值的方法需要大点的样本，但样本要多大好呢？我觉得应该是这三个其中一个。
