PLEASE HELP ME I'M LITERALLY GONNA CRY IF I GET THIS WRONG

A student adds a suspension of baker's yeast cells into bread dough. When this mixture is left in a large bowl undisturbed for several hours, the student returns to find that the bread dough has doubled in size. What gas molecule is responsible for this increase in dough size?
A. O2
B. N2
C. H2
D. CO2