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) $\frac{3}{2} \text{ D}$