Pure air is a homogeneous mixture which is made up of nitrogen and oxygen. Its elements are not actually separated from one another. But the composition of air is not uniform and may be effected by environmental factors, such as industrial pollution, heavy forestation or volcanic activity. In that case it can become heterogeneous mixture too
I don't remember this all that well but if I am thinking properly, it should be option C because Air is 2 oxygen molecules. That is what a homogeneous mixture is correct?
