Two spherical conductors A and B of radius a and b(b>a) are placed in air concentrically B is given charge +Q coulomb and A is grounded. The equivalent capacitance of these is
$$\begin{gathered} 1-4\mathrm{\pi }{\in }_0 \frac{\mathrm{ab}}{\mathrm{b}-\mathrm{a}} \\ 2-4\mathrm{\pi }{\in }_0 (\mathrm{a}+\mathrm{b}) \\ 3-4\mathrm{\pi }{\in }_0\mathrm{b} \\ 4-4\mathrm{\pi }{\in }_0\frac{{\mathrm{b}}^2}{\mathrm{b}-\mathrm{a}} \end{gathered}$$