\textbf{168-} An object is placed $20$ cm in front of a concave mirror whose radius of curvature is $40$ cm. If we move the object away from the mirror and go to very far distances, the image moves how many centimeters?
\hfill (1) $5$ \hfill (2) $20$ \hfill (3) $25$ \hfill (4) $15$