Solid $\mathrm{AgNO}{}_3$ is added to $0.005\mathrm{M}$ $\mathrm{K}{}_2\mathrm{CrO}{}_4$ (no volume change). With $K_{sp}(\mathrm{Ag}{}_2\mathrm{CrO}{}_4)=2.0\times 10^{-12}$, $\mathrm{Ag}{}_2\mathrm{CrO}{}_4$ starts to precipitate when $[\mathrm{Ag}{}^{+}]$ (in $\mathrm{mol}{\mathrm{L}}^{-1}$) reaches

1. A$2.0\times 10^{-5}\mathrm{M}$
2. B$4.0\times 10^{-10}\mathrm{M}$
3. C$8.0\times 10^{-8}\mathrm{M}$
4. D$2.0\times 10^{-10}\mathrm{M}$