A student wants to test the effect of caffeine on the heart rate of Daphnia, What is the control?
To do so the student needs a microscope to monitor the heart rate. He requires 6-10 days and should use spring water to dissolve the caffeine. Check out http://www.sciencebuddies.org/science-fair-projects/project_ideas/Zoo_p048.shtml#procedure for reference.
The control group would be the one where the caffeine is involved. Because that would make it the dependant variable because it would show you the outcome