A research aircraft measures the lower stratosphere at two places. Region X: ClO is about 1 ppb and ozone is very low. Region Y: ClO is negligible and ozone is normal. Which conclusion is best supported?

1. ARegion Y has more atomic Cl than X
2. BX has more $\mathrm{NO}{}_2$, which converts all ClO to $\mathrm{ClONO}{}_2$
3. CClO is a product of ozone photolysis, so low ozone must lower ClO
4. DChlorine is in an activated form ($\mathrm{Cl}$/$\mathrm{ClO}$) in X and is catalytically removing ozone there, while in Y chlorine is mostly tied up as $\mathrm{HCl}$ and $\mathrm{ClONO}{}_2$