Confidence Intervals of the Population Mean May Be Created for the Cases

Multiple Choice

Confidence intervals of the population mean may be created for the cases when the population standard deviation is known or unknown. How are these two cases treated differently?

A) Use the z table when s is unknown; use the t table when s is known.
B) Use the z table when s is known; use the t table when s is unknown.
C) Use the z table when σ is unknown; use the t table when σ is known.
D) Use the z table when σ is known; use the t table when σ is unknown.