The power of a lens (biconvex) is 1.25 m$$^{-1}$$ in particular medium. Refractive index of the lens is 1.5 and radii of curvature are 20 cm and 40 cm respectively. The refractive index of surrounding medium:

A

1.0

B

$$\frac{9}{7}$$

C

$$\frac{3}{2}$$

D

$$\frac{4}{3}$$