The focal length of an equi-convex lens is greater than the radius of curvature of any of the surfaces. Then the refractive index of the material of the lens is :

Question

(a) greater than zero but less than 1.5 (b) greater than 1.5 but less than 2.0 (c) greater than 2.0 but less than 2.5 (d) greater than 2.5 but less than 2.0