A lens is a transparent optical medium bounded by two surfaces; at least one of which should be spherical. Applying the formula of image formation by a single spherical surface successively at the two surfaces of a thin lens, a formula known as lens maker's formula and hence the basic lens formula can be obtained. The focal length (or power) of a lens depends on the radii of its surfaces and the refractive index of its material with respect to the surrounding medium. The refractive index of a material depends on the wavelength of light used. Combination of lenses helps us to obtain diverging or converging lenses of desired power and magnification. (i) A thin converging lens of focal length $20 \text{ cm}$ and a thin diverging lens of focal length $15 \text{ cm}$ are placed coaxially in contact. The power of the combination is (A) $-\frac{5}{6} \text{D}$ (B) $-\frac{5}{3} \text{D}$ (C) $\frac{4}{3} \text{D}$ (D) $\frac{3}{2} \text{D}$