Let's pretend that some students demonstrated the carbon cycle by observing

the reaction of yeast to sugar and salt. While salt (NaCl2) was not found to be

a food for yeast, sugar (C12H22O11) was. The resulting process is called

fermentation, in which the yeast took in sugar and oxygen to yield carbon

dioxide and alcohol.

In the experiment described above, what was the source of carbon dioxide

being released by yeast?