A thin plano-convex lens and a thin equi-concave lens are kept coaxially in contact as shown in the figure. Assuming both the lenses are made of glass of refractive index $\mu$, and R is the radius of curvature of each curved surface, the focal length of the combination is : (A) $\frac{\text{R}}{\mu - 1}$ (B) $-\frac{\text{R}}{\mu - 1}$ (C) $\frac{2\text{R}}{\mu - 1}$ (D) $-\frac{2\text{R}}{\mu - 1}$