Under Which of the Following Circumstances Is It Impossible to Construct

Short Answer

Under which of the following circumstances is it impossible to construct a confidence interval for the population mean? $\begin{array}{|l|l|}\hline A&\text {A non-normal population with a large sample and an unknown population variance. }\\\hline B&\text {A normal population with a large sample and a known population variance. }\\\hline C&\text {A non-normal population with a small sample and an unknown population variance. }\\\hline D&\text { A normal population with a small sample and an unknown population variance.}\\\hline \end{array}$