Correct Response: C. Cyanobacteria are capable of photosynthesis and release oxygen gas as a byproduct of the photosynthetic process. Fossil evidence indicates the presence of cyanobacteria during the early stages of the earth, so they had the potential to be responsible for the increase of atmospheric oxygen. Gases escaping from volcanoes (A) are sulfur-based, so volcanoes did not contribute to the initial increase in oxygen. Multicellular organisms (B) emerged later than cyanobacteria. In addition, many multicellular organisms, such as animals, do not release oxygen to the atmosphere. The condensation of water (D) is part of the water cycle; it does not produce oxygen during the process of changing physical states.
