$\mathrm{v}=\frac{\mathrm{d} \mathrm{x}}{\mathrm{dt}}=\mathrm{A} \omega \cos \omega \mathrm{t}=\omega \sqrt{\mathrm{A}^{2}-\mathrm{x}^{2}}$
$\mathrm{a}=\frac{\mathrm{d} \mathrm{v}}{\mathrm{dt}}=-\mathrm{A} \omega^{2} \sin \omega \mathrm{t}$
$\mathrm{a}=\frac{\mathrm{d} \mathrm{v}}{\mathrm{dt}}=-\omega^{2} \mathrm{x}$
But $x=-\frac{a}{\omega^{2}}$
$\therefore \quad v=\omega \sqrt{A^{2}-\frac{a^{2}}{\omega^{4}}}$
or $v^{2}=\omega^{2}\left(A^{2}-\frac{a^{2}}{\omega^{4}}\right)$
($A$) The amplitude of oscillation in the first case changes by a factor of $\sqrt{\frac{M}{m+M}}$, whereas in the second case it remains unchanged
($B$) The final time period of oscillation in both the cases is same
($C$) The total energy decreases in both the cases
($D$) The instantaneous speed at $x_0$ of the combined masses decreases in both the cases