[ it's okay guys, Kiryu still loves you even if you don't know that the mitochondria is the powerhouse of the cell. ]

Well, carbon dioxide and nitrogen aren't used by mitochondria. I don't think nitrogen is used in cells, and carbon dioxide is usually released by cells, it doesn't get used. That's why we get sick if we breathe in too much smoke.

[ He really is trying to explain it so she'll learn... ]

So your choices are DNA and ATP. DNA is kind of like... the instruction manual for the cell, so that leaves...?