\textbf{173.} An object is placed 120 cm in front of a lens and the magnification of the lens is $0.4$. How should the object be moved along the principal axis so that the image length becomes half the object length?
\begin{enumerate}
\item[(1)] Move 30 cm away from the lens.
\item[(2)] Move 20 cm closer to the lens.
\item[(3)] Move 40 cm away from the lens.
\item[(4)] Move 40 cm closer to the lens.
\end{enumerate}