A Random Sample of Size N Has Been Selected from a Normally

Multiple Choice

A random sample of size n has been selected from a normally distributed population whose standard deviation is s. In estimating an interval for the population mean, the t-distribution should be used instead of the z-test if:

A) n ?
30 and $\sigma$ is known.
B) $\sigma$ is unknown and estimated by s, and the population is normal.
C) Population is normal and $\sigma$ is known.
D) None of these choices are correct.