1. When performing a test of significance about a population mean, a t distribution in- stead of a normal (z) distribution often is used. Which of the following is the most appropriate explanation for this?

a. The population standard deviation is unknown.

b. The sample does not follow a normal distribution.

c. There is an increase in the variability of the test statistic due to the estimation of the population standard deviation.

d. The sample standard deviation is unknown.