$K_{sp}$ of $\mathrm{PbSO}{}_4$ is $1.6\times 10^{-8}$. Its solubility in $0.04\mathrm{M}$ $\mathrm{Na}{}_2\mathrm{SO}{}_4$ solution (in $\mathrm{mol}{\mathrm{L}}^{-1}$) is approximately

1. A$1.3\times 10^{-4}\mathrm{M}$
2. B$2.0\times 10^{-7}\mathrm{M}$
3. C$1.0\times 10^{-5}\mathrm{M}$
4. D$4.0\times 10^{-7}\mathrm{M}$