Large amounts of oxygen gas appeared in earth's atmosphere about 2.7 bya years ago, a. after photo-autotrophic prokaryotes evolved b. after eukaryotes evolved c. before anaerobic organisms evolved d. after the first organisms colonized land

Why are carbon dioxide concentrations expected to increase? a carbon dioxide concentrations are expected to increase, because fossil fuel burning is expected to increase b. carbon dioxide concentrations are expected to increase, because of an increase in agriculture. c.carbon dioxide concentrations are expected to increase, because more land will have to be cleared for increasing populations and agricultural use. d. all of the above select the best answer from the choices provided mark this and return save and exit next submit submit