Enhancing Reliability in Yeast Fermentation Experiment

In a recent study examining the impact of varying sugar concentrations on yeast fermentation, researchers designed an experiment to test how different amounts of sugar would influence the rate of carbon dioxide production. They set up five flasks, each containing the same amount of yeast and water but with different sugar concentrations: 0%, 1%, 5%, 10%, and 15%. After allowing the mixture to ferment for 24 hours, the researchers measured the volume of carbon dioxide produced in each flask.

Which of the following statements reflects a key design element that enhances the reliability of this experiment?