A plano convex lens fits exactly into a plano concave lens. Their plane surfaces are parallel to each other. If lenses are made of different materials of refractive indices and and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is R (a) (b) (c) (d) 2(H-H) R (H-H) 2R (H-H) R 2(+H)