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 1 and 2 and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is

1. A. R 2( 1 + 2)
2. B. R 2( 1 - 2)
3. C. R ( 1 - 2)
4. D. 2R ( 2 - 1)

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