Since the experiment is about testing the temperature; the temperature would be the independent variable, not the controlled variable. A water bath would be used in this experiment to set up different increments of the independent variable.
When investigating the effect of temperature on photosynthesis, the other limiting factors (concentration of carbon dioxide and light intensity) must be controlled. The light intensity can be controlled by placing the test tube containing the Elodea at a constant distance from the same light source for each trial (Choice C) to make sure that the light intensity is the same for all trials.
Using the same volume and concentration of sodium hydrogen carbonate solution (Choice B and Choice D)ensures that the source of carbon dioxide is controlled, and it is not a limiting factor.
