The radii of curvature of two surfaces of a convex lens are $R$ and $2R$. If the focal length of this lens is $\left(\frac{4}{3}\right)R$, the refractive index of the material of the lens is : (A) $\frac{5}{3}$ (B) $\frac{4}{3}$ (C) $\frac{3}{2}$ (D) $\frac{7}{5}$