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 s.

(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.