Where did the oxygen in Earth's atmosphere come from?

The answer to 1) is somewhere in the course materials. The other 2 require you to think about what you have learned and apply it to the question. I recall one theory that said the plants had separated the O2 from the CO2 just as they do now. The CO2 was a by-product of some life form that lived on a sulphur enriched environment, much to its detriment. Nowhere near enough answer space to handle the other 2 questions.