One reason for using a t distribution instead of the standard Normal curve to find critical values when calculating a level C confidence interval for a population mean is that
(a) z can be used only for large samples.
(b) z requires that you know the population standard deviation σ.
(c) z requires that you can regard your data as an SRS from the population.
(d) z requires that the sample size is at most 10% of the population size.
(e) a z critical value will lead to a wider interval than a t critical value.