Two identical springs are attached to a small block P. The other ends of the springs are fixed at A and B. When P is in equilibrium the extension of top spring is 20 cm and extension of bottom spring is l0 cm. The period of small vertical oscillations of P about its equilibrium position is (use g = 9.8 $\mathrm{m}/{\mathrm{s}}^2$)

• A

  $\frac{2\mathrm{\pi }}{7}\sec$

• B

  $\frac{\mathrm{\pi }}{7}\sec$

• C

  $\frac{2\mathrm{\pi }}{5}\sec$

• D

  none of these