The problem with (A) is that it is too strong a statement. While the passage does suggest an equilibrium in ozone amounts without the presence of chlorine, what (A) suggests is that chlorine is necessary for the destruction of ozone. But there could be other elements that also similarly facilitate the destruction of ozone. We can't infer that it is ONLY chlorine that destroys ozone from the passage.
As for why (D) is the correct answer, we know that regulating the use of CFCs will help slow down the depletion of the ozone layer, as discussed in the second paragraph. We also know that the ozone layer blocks out a lot of UV radiation (lines 3-5), and that this UV radiation contributes to skin cancer (lines 1-2). Thus, we can conclude that regulating CFC use will indirectly help lower skin cancer rates. Hope this helps!
