$\mathrm{AgNO}{}_3$ is added gradually to a solution that is $0.020\mathrm{M}$ in $\mathrm{Cl}{}^{-}$ (no volume change). With $K_{sp}(\mathrm{AgCl})=1.8\times 10^{-10}$, at what $[\mathrm{Ag}{}^{+}]$ does $\mathrm{AgCl}$ just begin to precipitate?

1. A$1.3\times 10^{-5}$
2. B$3.6\times 10^{-12}$
3. C$1.8\times 10^{-8}$
4. D$9.0\times 10^{-9}$