Which of the following statements about ozone is correct?

The Montreal Protocol is aimed at:

The thickness of the ozone layer is measured in terms of:

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 one of the following is measured in Dobson units $(DU)$?

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.