Two springs of equal length but having stiffness of 10 N/mm and 15 N/mm support a mass of 2 tonnes in series. Find the frequency of vibration

A

$$\frac{1}{2\pi}\sqrt{3} Hz$$

B

$$\frac{1}{2\pi}\sqrt{10} Hz$$

C

$$\frac{1}{2\pi}\sqrt{15} Hz$$

D

$$\frac{1}{2\pi}\sqrt{2} Hz$$