A point object is placed on the principal axis of a concave mirror quite far away from the pole and moved at a constant speed 0.5 cm/sec towards the pole. Its image also moves. It is found that the object and the image cross each other at a point which is at a distance 50 cm from the pole. Focal length of the mirror is -
(A) 50 cm
(B) 35 cm
(C) 25 cm
(D) 15 cm
