The structure labeled A represents: 
Definitions:
Species Richness
A measure of the number of different species present in an ecosystem, providing an indication of biodiversity.
Montreal Protocol
International agreement designed to protect the ozone layer by gradually eliminating the production of ozone-depleting substances.
CFCs
Chlorofluorocarbons, synthetic organic compounds used in air conditioning, refrigerants, and aerosols, which contribute to ozone layer depletion.
Global Warming Chemicals
Substances contributing to the greenhouse effect and global warming, typically including carbon dioxide, methane, nitrous oxide, and fluorinated gases.
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