A piano convex lens fits exactly into a piano concave lens. Their plane surface are parallel to each other. If the lenses are made of different materials of refractive indices \[{{\mu }_{1}}\] & \[{{\mu }_{2}}\] and R is the radius of curvature of the curved surface of the lenses, then focal length of combination is

• A. \[\frac{R}{{{\mu }_{1}}-{{\mu }_{2}}}\]
• B. \[\frac{2R}{{{\mu }_{1}}-{{\mu }_{2}}}\]
• C. \[\,\frac{R}{2\left( {{\mu }_{1}}-{{\mu }_{2}} \right)}\]
• D. \[\,\frac{R}{2-\left( {{\mu }_{1}}+{{\mu }_{2}} \right)}\]