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 $\mu_1$ and $\mu_2$ and R is the radius of curvature of the curved surface of the lenses, then the focal length of the combination is -
( A ) $\frac{2R}{(\mu_2 - \mu_1)}$
( B ) $\frac{R}{(\mu_1 - \mu_2)}$
( C ) $\frac{R}{2(\mu_1 + \mu_2)}$
( D ) $\frac{R}{2(\mu_1 - \mu_2)}$