$\mathrm{AgNO}{}_3$ is added gradually to a solution containing $0.010\mathrm{M}$ $\mathrm{Cl}{}^{-}$ and $0.010\mathrm{M}$ $\mathrm{CrO}{}_4{}^{2-}$. $K_{sp}$: $\mathrm{AgCl}=1.8\times 10^{-10}$, $\mathrm{Ag}{}_2\mathrm{CrO}{}_4=2.0\times 10^{-12}$. When $\mathrm{Ag}{}_2\mathrm{CrO}{}_4$ just begins to precipitate, $[\mathrm{Cl}{}^{-}]$ left in solution (in $\mathrm{mol}{\mathrm{L}}^{-1}$) is

1. A$1.8\times 10^{-8}\mathrm{M}$
2. B$1.4\times 10^{-5}\mathrm{M}$
3. C$2.0\times 10^{-10}\mathrm{M}$
4. D$1.3\times 10^{-5}\mathrm{M}$