\textbf{172.} A simple pendulum with length $80\,\text{cm}$ has a small damping. The pendulum is oscillating. How should we change the length of the pendulum so that its period of oscillation becomes half?
\begin{itemize}
\item[(1)] Decrease by 60 cm. \hfill (2) Increase by 60 cm.
\item[(3)] Decrease by 20 cm. \hfill (4) Increase by 20 cm.
\end{itemize}