An object is moving towards a concave mirror of focal length 24 cm. When it is at a distance of 60 cm from the mirror, its speed is 9 cm/s. The speed of its image at that instant, is
a. 4 cm/s towards the mirror
b. 6 cm/s towards the mirror
c. 4 cm/s away from the mirror
d. 6 cm/s away from the mirror