All of these are natural events that could alter an ecosystem EXCEPT - A: burning of fossil fuel
B: wildfires
C: floods
D: volcanic activity

A) Burning of fossil fuels

Explanation:
Burning fossil fuels is an anthropogenic activity, meaning humans do it. Even though humans can create wildfires, for the most part it is a natural and good thing for soil; humus - dead organic matter - is good for top soil. Flood and Volcanic eruption are both done by mother nature.

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Leaves specifically the gas exchange between plants and the environment. while plants undergo photosynthesis, they also partake in cellular respiration. respiration is the process that happens in the leaves of plants.