A plano-convex lens fits exactly into a plano-concave lens. Their plane surfaces are parallel to each other. If the lenses are made of different material of refractive indices `mu_(1)` and `mu_(2)` and R is the radius of curvature of the curved surface of the lenses, then focal length of the combination is
A. `(R)/(2(mu_(1)+mu_(2)))`
B. `(R)/(2(mu_(1)-mu_(2)))`
C. `(R)/((mu_(1)-mu_(2)))`
D. `(R)/((mu_(1)+mu_(2)))`