A small spherical steel ball is placed at a distance slightly away from the center of a concave mirror having radius of curvature $250 \mathrm{~cm}$. If the ball is released, it will now move on the curved surface. What will be the periodic time of this motion? Ignore frictional force and take $\mathrm{g}=10 \mathrm{~m} / \mathrm{s}^{2}$.
(A) $(\pi / 4) \mathrm{s}$
(B) $\pi \mathrm{s}$
(C) $(\pi / 2) \mathrm{s}$
(D) $2 \pi \mathrm{s}$