A point object on the principal axis at a distance 15 cm in front of a concave mirror of radius of curvature 20 cm has velocity 2 mm/s perpendicular to the principal axis. The magnitude of velocity of image at that instant will be:

(1) 2 mm/s
(2) 4 mm/s
(3) 8 mm/s
(4) 16 mm/s