Two students discuss CFC-12. Student A: the Cl released arises from the C-F bond. Student B: it arises from the C-Cl bond on absorption of UV in the stratosphere, and the Cl then reacts with $\mathrm{O}{}_3$. Student C: the Cl is released by reaction with water in the troposphere. Which is correct, and what is the first reaction of the released Cl?

1. AStudent A; $\mathrm{Cl}+\mathrm{O}{}_3\to \mathrm{ClO}+\mathrm{O}{}_2$
2. BStudent B; $\mathrm{Cl}+\mathrm{O}{}_3\to \mathrm{ClO}+\mathrm{O}{}_2$
3. CStudent C; $\mathrm{Cl}+\mathrm{O}{}_2\to \mathrm{ClO}{}_2$
4. DStudent B; $\mathrm{Cl}+\mathrm{O}{}_3\to \mathrm{ClO}{}_3$