An experiment was designed to investigate the effect of caffeine on the heartbeat of water fleas. Two populations of water fleas were cultured. Both populations were in water with the same mineral content, were supplied with identical amounts of bacteria as food, received the same amount of light, and had their temperature maintained at 20 degrees C. Every two hours water fleas from both populations were selected and their heart beats were monitored. The fleas from population 1, however, had caffeine administered 5 minutes before the heartbeat was checked, while population 2 was given nothing. What is a controlled variable?

That is administered and controlled

Step-by-step explanation:

A variable is the condition or a product that may vary without the influence of any investigator. Such variables can be found abundantly in our daily lives, ranging from air to sunlight. Controlling such variables ensures the limitation of their influence on the outcome of an experiment. In connection to this experiment, the variables that are controlled are the mineral content in the water, the amount of bacteria food, the amount of light and the the constant temperature of 20 degrees C.

If not administered or these variables controlled, the experiment would not clearly explain the impact of caffeine on the heart beat of the water fleas as other variables may have influenced it. Thus variables are controlled to study the singular impact of any agent or effect.