A point object $P$ is placed at centre of curvature of a concave mirror of focal length $25 \mathrm{~cm}$. The mirror is cut into two halves and shifted symmetrically $1 \mathrm{~cm}$ apart in perpendicular to the optical axis. The distance between images formed by both parts is:
(a) $2 \mathrm{~cm}$
(b) $1 \mathrm{~cm}$
(c) $3 \mathrm{~cm}$
(d) $4 \mathrm{~cm}$