A Sample of 20 Items from a Population with an Unknown

Multiple Choice

A sample of 20 items from a population with an unknown $\sigma$ is selected in order to develop an interval estimate of $\mu$ . Which of the following is not necessary?

A) We must assume the population has a normal distribution.
B) We must use a t distribution.
C) Sample standard deviation must be used to estimate $\sigma$ .
D) The sample must have a normal distribution.