\textbf{159.} A concave mirror has its object placed on the principal axis and the image formed is half the size of the object. If we move the object 24 cm closer to the mirror, the image forms at infinity. What is the radius of this mirror in centimeters?
\[
(4)\ 36 \qquad (3)\ 24 \qquad (2)\ 18 \qquad (1)\ 12
\]