A vertical spring of natural length $$l_0$$ and force constant K is stretched by l when a mass m is suspended from its lower end. By pulling the mass down a little the system is left off to oscillate. The time period of oscillation is

A

$$2\pi\sqrt{\frac{l_0}{g}}$$

B

$$2\pi\sqrt{\frac{l}{g}}$$

C

$$2\pi\sqrt{\frac{l_0 + l}{g}}$$

D

$$2\pi\sqrt{\frac{K}{m}}$$