In oxidation, oxygen is needed to create a chemical reaction.
When a human dies, his corpses gets decomposed by decomposing bacteria. What's produced by the decomposing bacteria is the "fertilizer" for plants.
You can determine that the igneous intrusion is younger than layer C, because layer C is the oldest, acknowledging the fact that it is the deepest layer.
The semicircles are probably the longitude and the latitude, if that what you are talking about.