Answer the following question given this paragraph: The oxygen atoms, however, form oxygen gas, which is a waste product of photosynthesis. The oxygen given off supplies most of the oxygen in our atmosphere. Before photosynthesis evolved, Earth's atmosphere lacked oxygen altogether, and this highly reactive gas was toxic to the many organisms living at the time. Something had to change! Most contemporary organisms rely on oxygen for efficient respiration. So plants don't just "restore" the air, they also had a major role in creating it!. Q: Plants not only contribute food but what else for organisms? Choices: - dioxide - hydrogen - carbon - oxygen A:
A:
oxygen