Answer:
A.The hole in the ozone layer
Step-by-step explanation:
- Montreal protocol is an international treaty that was signed to control the use of substances that are resulting in the depletion of the ozone hole.
- This treaty was signed in the year 1987 and became effective form 1989.
- Since the time that this treaty has come into force, there has been a significant improvement in the ozone hole that was observed over Antarctic as it's size has decreased.
- This improvement has been possible because the treaty helped to phase out chemicals such as chlorofluorocarbons, carbon tetrachloride, etc. that are responsible for ozone hole depletion.