A system of lenses is formed by fitting a plans convex lens, whose radius of curvature is R and $\mu_1$,the refractive index of plano convex lens is $\large\frac{3}{2} $ and $\mu_2$ the refractive index of Plano concave lens is $\large\frac{4}{3}$. Then the focal length of combination is

Question

$(a)\;2R \\ (b)\;\frac{R}{2} \\ (c)\;6R \\ (d)\;\frac{R}{6} $