Well too much oxygen can kill humans which is one. And 2 If the whole atmosphere was oxygen then things will burn easily.
Number 3 is the answer to this.
The right answer here is option C. They occur in areas with ancient, mineral-poor soil.
An example of that is Amazonia in Brazil, it's one of the biggest forests on earth, and at the same time, we know its soil is poor, but at the same time it has some special materials that can be found there, such as niobium. This forest is, too, rainy almost all the time, and this many trees maintain the temperature of the whole earth stabilized. These kinds of forests can grow in this soil because of the burlap, that's organic materials from its own trees. It's consumed by them, and through this way, it survives and extends its size when humans don't use its resources too much.
