What gas was present in earth's earliest atmosphere and used by cyanobacteria during photosynthesis?
a.
oxygen
b.
carbon dioxide
c.
nitrogen
d.
methane

These microbes conduct photosynthesis: using sunshine, water and carbon dioxide to produce carbohydrates and, yes, oxygen. In fact, all the plants on Earth incorporate symbiotic cyanobacteria (aka chloroplasts) to do their photosynthesis for them down to this day. so i believe  it is carbon dioxide.

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