In Constructing a Confidence Interval for the Population Mean When $\bar { x } \pm t _ { a / 2 } \frac { s } { \sqrt { n } }$

In constructing a confidence interval for the population mean when the population variance is unknown, which of the following assumptions is required when using the following formula? $\bar { x } \pm t _ { a / 2 } \frac { s } { \sqrt { n } }$ $\begin{array}{|l|l|}\hline A&\text {The sample size is greater than 30 . }\\\hline B&\text {The population variance is known. }\\\hline C&\text { The population is normal.}\\\hline D&\text {The sample is drawn from a positively skewed distribution. }\\\hline \end{array}$