An experiment was designed to investigate the effect of caffeine on the rate of heartbeat of fruit flies. Two populations of fruit flies were grown in a lab. Both
populations were raised drinking the same water and the same type of food. Both
populations were exposed to the same amount of light and temperature at 20
degrees Celsius. The fruit flies' heartbeats were then monitored at two hour
intervals. The fruit flies of population one were given caffeine five minutes before
their heartbeat was checked. The flies of population two were not given any
caffeine.
Which of the following is a controlled variable in this experiment?
temperature
caffeine
heart rate
experimental group