Solid $\mathrm{AgNO}{}_3$ is added slowly to a solution that is $0.01\mathrm{M}$ in both $\mathrm{Cl}{}^{-}$ and $\mathrm{I}{}^{-}$. $K_{sp}$: $\mathrm{AgCl}=1.8\times 10^{-10}$, $\mathrm{AgI}=8.3\times 10^{-17}$. When $\mathrm{AgCl}$ just starts to precipitate, $[\mathrm{I}{}^{-}]$ remaining in solution (in $\mathrm{mol}{\mathrm{L}}^{-1}$) is

1. A$1.8\times 10^{-8}\mathrm{M}$
2. B$4.6\times 10^{-9}\mathrm{M}$
3. C$8.3\times 10^{-15}\mathrm{M}$
4. D$4.6\times 10^{-6}\mathrm{M}$