One end of a mass less spring having force constant $\mathrm{k}$ and length $50 \mathrm{~cm}$ is attached at the upper end of a plane inclined at an angle $e=30^{\circ} .$ When a body of mass $m=1.5 \mathrm{~kg}$ is attached at the lower end of the spring, the length of the spring increases by $2.5 \mathrm{~cm}$. Now, if the mass is displaced by a small amount and released, the amplitude of the resultant oscillation is .........
(A) $(\pi / 7)$
(B) $(2 \pi / 7)$
(C) $(\pi / 5)$
(D) $(2 \pi / 5)$