If the distance between object and its two times magnified virtual image produced by a curved mirror is \( 15 \, \text{cm} \), the focal length of the mirror must be:
\( -12 \, \text{cm} \)
\( 15 \, \text{cm} \)
\( {10}/{3} \, \text{cm} \)
\( -10 \, \text{cm} \)