As shown in the figure, a block of mass m is hung from the ceiling by the system of springs consisting of two layers. The force constant of each of the springs is k. The frequency of the vertical oscillations of the block is

A

$$\frac{1}{2\pi}\sqrt{\frac{k}{5m}}$$

B

$$\frac{1}{2\pi}\sqrt{\frac{4k}{5m}}$$

C

$$\frac{1}{2\pi}\sqrt{\frac{5k}{6m}}$$

D

$$\frac{1}{2\pi}\sqrt{\frac{6k}{5m}}$$