$A$ decrease in the concentration of which gas in the atmosphere leads to an increase in the incidence of skin cancer?

Assertion : $UV$ radiation causes photodissociation of ozone into $O_2$ and $O$,thus causing damage to the stratospheric ozone layer.
Reason : Ozone hole is resulting in global warming and climate change

Which of the following statement$(s)$ is/are correct?
$(i)$ The Montreal Protocol was signed in $1987$ and became effective in $1989$.
$(ii)$ 'Good' ozone is found in the troposphere.
$(iii)$ Ozone is formed by the action of $UV$ rays on molecular oxygen and degrades into molecular oxygen.
$(iv)$ $CFCs$ are responsible for the imbalance between the production and degradation of ozone.
$(v)$ $DNA$ and proteins of living organisms absorb $UV$ rays.

In the stratosphere,which of the following elements acts as a catalyst in the degradation of ozone and the release of molecular oxygen?

Ozone is used in swimming pools because

The thickness of ozone in a column of air in the atmosphere is measured in terms of: