Ms. green's biology students were studying cellular respiration. using baking yeast to test the rate of fermentation, student groups mixed yeast with different manipulated variables. the groups used the height of the carbon dioxide bubbles produced in a test tube to determine which variable(s) produced the highest rate of fermentation. students hypothesized that adding a sixth test tube would produce the greatest amount of carbon dioxide.  the answer is warm water, yeast, and lactose.