Please help me on 9th grade bio hw!!On yeast fermentation/cell.resp.?

We did a lab where we tested teh amount of CO2 that was made when yeast was mixed together with distilled water, fructose, sucrose, and other things. Now we have to answer some questions and I don't get one of them which is "Why is fermentation occurring and not complete cellular respiration?" Please help me!!! Thanks.