$50\mathrm{mL}$ of $0.20\mathrm{M}$ $\mathrm{Ba}(\mathrm{NO}{}_3){}_2$ is mixed with $50\mathrm{mL}$ of $0.20\mathrm{M}$ $\mathrm{Sr}(\mathrm{NO}{}_3){}_2$. Solid $\mathrm{K}{}_2\mathrm{CrO}{}_4$ is then added slowly (negligible volume change). $K_{sp}(\mathrm{BaCrO}{}_4)=1.2\times 10^{-10}$, $K_{sp}(\mathrm{SrCrO}{}_4)=3.6\times 10^{-5}$. What is $[\mathrm{Ba}{}^{2+}]$ at the moment $\mathrm{SrCrO}{}_4$ just begins to precipitate?

1. A$3.3\times 10^{-7}$
2. B$1.2\times 10^{-9}$
3. C$6.7\times 10^{-7}$
4. D$1.1\times 10^{-5}$