A student investigated the effect of caffeine concentration on the heart rate of Daphnia (water fleas).
The student used five different concentrations of caffeine solution: 0.1%, 0.2%, 0.3%, 0.4% and 0.5%.
For each concentration, a single Daphnia was transferred to a cavity slide containing 2 cm3 of the caffeine solution.
The heart rate of the Daphnia (in beats per minute) was recorded under a light microscope after an acclimation period of 5 minutes5\text{ minutes}5 minutes. The temperature of the slide was kept constant at 22 ∘C22\text{ }^{\circ}\text{C}22 ∘C.
Describe how to set up a control for this investigation.
