According to the researchers, which human activity is MOST likely to be a contributing factor in global warming? A) Humans have added carbon dioxide to the atmosphere by burning fossil fuels. B) Humans have released vast quantities of chemicals that have created excess ozone in the upper atmosphere. C) Humans have changed the chemistry of the world's oceans by the use of phosphates and nitrogen-containing compounds. D) Humans have created acid rain by releasing sulfuric acids into the air in the exhaust from factories and power plants.