Which of the following is effectively used to remove E. coli bacteria from water?

What is order and molecularity of following reaction?

$$ \mathrm{NO}_{2(\mathrm{~g})}+\mathrm{NO}_{2(\mathrm{~g})} \longrightarrow 2 \mathrm{NO}_{(\mathrm{g})}+\mathrm{O}_{2(\mathrm{~g})} $$

The solubility of salt $\mathrm{BA}_2$ is $4 \times 10^{-4} \mathrm{~mol} \mathrm{dm}^{-3}$.

What is solubility product of the salt?

Which among the following compounds has lowest boiling point?