Solve this :

25. A frictionless tunnel is made along the chord AB of the earth as shown. An object of mass m is released at point B from rest. Time period of oscillation of the object along the chord is :


$$\begin{gathered} \left(1\right) \frac{\mathrm{\pi }}{2}\sqrt{\frac{\mathrm{R}}{\mathrm{g}}} \left(2\right) \mathrm{\pi }\sqrt{\frac{\mathrm{R}}{\mathrm{g}}} \\ \left(3\right) \sqrt{\frac{\mathrm{R}}{\mathrm{g}}} \left(4\right) 2\mathrm{\pi }\sqrt{\frac{\mathrm{R}}{\mathrm{g}}} \end{gathered}$$