Solid $\mathrm{NaCl}$ is added gradually (no volume change) to $0.002\mathrm{M}$ $\mathrm{AgNO}{}_3$. Given $K_{sp}(\mathrm{AgCl})=1.8\times 10^{-10}$, precipitation just begins when $[\mathrm{Cl}{}^{-}]$ (in $\mathrm{mol}{\mathrm{L}}^{-1}$) reaches

1. A$1.8\times 10^{-8}\mathrm{M}$
2. B$9.0\times 10^{-8}\mathrm{M}$
3. C$1.3\times 10^{-5}\mathrm{M}$
4. D$3.6\times 10^{-13}\mathrm{M}$