Based on option A the transition is just more trees, which gives out more oxygen. B having a massive volcanic eruption can cause certian things to die off and could eventually kill off certian species and forms of air. Example would be killing of the trees. C oil spill in the bay can cause damage by pollution and dieing of species. D acres- wide forest fire. Again that would take away oxygen supply and take away natural habbitates of animals/species. There for the answer would be A.
