A plano-convex lens (focal length `f_2`, refractive index`mu_(2)`, radius of curvature R) fits exactly into a plano-concave lens (focal length`f_(1)` refractive index `mu_1`, radius of curvature R). Their plane surfaces are parallel to each other. Then, the focal length of the combination will be :
A. `f_(1)-f_(2)`
B. `(R)/(mu_(2)-mu_(1)`
C. `(2f_1f_(2))/(f_(1)-f_(2)`
D. `f_(1)+f_(2)`
