What caused the composition of Earth’s early oceans to transition from dissolved carbon dioxide to dissolved oxygen?

A.
Ultraviolet rays from the Sun converted dissolved carbon dioxide to oxygen.
B.
Excessive rainfall over ocean waters caused carbon dioxide to separate into carbon and oxygen.
C.
Cyanobacteria growing in ocean waters used up carbon dioxide and released oxygen.
D.
Lightning caused the dissolved carbon dioxide in the ocean to decay into oxygen.