Biology test review question,im stuck on this.?

To study the effect of common substances on the heart rate of Daphnia, students place a Daphnia in a drop of water on a gl slide. The students then added 1 or more drops of a test substance dissolved in water to the slide, waited 10 seconds, then counted heart beats for 10 seconds. The students used a clean slide and a new Daphnia each time. The substances tested were regular tap water, coffee, and ethanol (an alcohol). Identify the independent and dependent variables, the control group and the experimental groups.