\textbf{166.} An object is placed at a distance of $15\,\text{cm}$ in front of a concave mirror with radius $40\,\text{cm}$, and the image length is $4\,\text{cm}$. How many centimeters should the object be moved so that the image also has a length of $4\,\text{cm}$?
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(1) $5$ \hspace{1cm} (2) $15$ \hspace{1cm} (3) $25$ \hspace{1cm} (4) $55$
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