WILL GIVE BRAINLIEST Yeast is a fungus which grows inside humans digestive and reproductive track. When the yeast is in normal limits it helps to keep the bacteria living in humans in balance. A balanced human maintains homeostasis and is healthy.
What is the relationship between yeast and humans?

An important feature of these yeasts that makes them such useful organisms for studying biological processes in humans, is that their cells, like ours, have a nucleus containing DNA packaged into chromosomes. ... Yeast cells divide in a similar manner to our own cells.

I would think mutualism.

Explanation:

Because the yeast is provided a place to grow, and the human is benefitted by the yeast helping to keep balance.

A. nitrogen oxide 

or c.sulfur dioxide