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 materials of refractive indices ${\mathrm{\mu }}_1 \mathrm{and} {\mathrm{\mu }}_2$ and ‘R’ is the radius of curvature of the curved surfaces of the lenses then the focal length of the combination is 

• A

  $\frac{\mathrm{R}}{{\mathrm{\mu }}_1-{\mathrm{\mu }}_2}$

• B

  $\frac{2\mathrm{R}}{{\mathrm{\mu }}_1-{\mathrm{\mu }}_2}$

• C

  $\frac{\mathrm{R}}{2\left({\mathrm{\mu }}_1-{\mathrm{\mu }}_2\right)}$

• D

  $\frac{\mathrm{R}}{{\mathrm{\mu }}_1+{\mathrm{\mu }}_2}$