The Chemical Composition of Ancient Sediments Suggests That Oxygen Was

The chemical composition of ancient sediments suggests that oxygen was essentially absent from the atmosphere of the early Earth; how can its abundance in today's atmosphere be explained?

A) Sunlight split water molecules apart, a process that produced oxygen gas.
B) Volcanic activity released oxygen that had been trapped beneath the Earth's surface.
C) Cyanobacteria and some eukaryotes produced oxygen as a by-product of photosynthesis.
D) Chemical erosion of the Earth's surface released oxygen initially bound in surface rocks.