\textbf{121-} In manufacturing a cone in the shape of a right circular cone with volume $\dfrac{\pi}{3}$, at what height is the least material consumed?
\hspace*{2cm} (1) $\dfrac{\sqrt{2}}{2}$ \hspace{2cm} (2) $1$ \hspace{2cm} (3) $\sqrt[4]{2}$ \hspace{2cm} (4) $\sqrt{2}$