A concentrated mass m is attached at centre of a rod of length 2 L as shown in figure. The rod is kept in a horizontal equilibrium position by a spring of stiffness 4 k. For very small amplitude of vibration, neglecting the weight of the rod and spring, the undamped natural frequency ($$w_n$$) of the system is

A

$$\surd(2 k/m)$$

B

$$\surd(4 k/m)$$

C

$$\surd(8 k/m)$$

D

$$\surd(16 k/m)$$