A student is conducting an investigation to determine the effect of temperature on the metabolism of yeast (Saccharomyces cerevisiae). Yeast and sugar are added to water, the gas produced is captured, and its volume is recorded. During the experiment, which variables should be held constant?
A. Mass of sugar and water temperature.
B. Mass of yeast and mass of sugar.
C. Volume of gas and water temperature.
D. Volume of gas and mass of yeast.