A spherical capacitor is made of two thin concentric metal shells of radii $9\ \text{cm}$ and $10\ \text{cm}$, with air between them. The outer shell is earthed. Take $\dfrac{1}{4\pi\varepsilon_0} = 9 \times 10^9\ \text{N m}^2\,\text{C}^{-2}$. Its capacitance is