Q72

The following is the incomplete operation table of a $4-$element group.

$$\begin{array}{|l|l|l|l|l|} \hline \textbf{*} & \textbf{e}& \textbf{a} &\textbf{b} & \textbf{c}\\\hline \textbf{e} & \text{e}& \text{a} & \text{b} & \text{c} \\\hline \textbf{a} & \text{a}& \text{b} & \text{c} & \text{e}\\\hline \textbf{b}\\\hline \textbf{c} \\\hline\end{array}$$

The last row of the table is