An experiment was designed to investigate the effect of caffeine on the heartbeat of Daphnia magna, or water fleas. These tiny, aquatic animals are used because their transparent bodies make it easy to see their hearts and measure their heart rates. Two populations of Daphnia were raised, (A) and (B). Both populations lived in tanks of water at the same temperature and received identical amounts of food. Every 2 hours, water fleas from both populations were selected and their heartbeats were monitored. Population (A) had 0.5 g of caffeine given to them five minutes before their heartbeat was checked. Population (B) were given 0.5 g of water and with no caffeine.

This question is incomplete. It is asking to identify the following in the experiment:

Independent variable: CAFFEINE

Dependent variable: HEARTBEAT

Experimental group: POPULATION A, which were given 0.5g of caffeine

Control group: POPULATION B, which were not given caffeine

Constants: SAME TEMPERATURE, SAME AMOUNT OF FOOD, SAME TIME BEFORE CHECKING

Explanation:

Independent variable in an experiment is the variable which is changed or manipulated by the experiment. In this experiment, the independent variable is the CAFFEINE.

Dependent variable is the variable that is measured by the experiment or in other words, the variable that responds to changes made to the independent variable. In this experiment, the dependent variable is the HEARTBEAT measured.

The experimental group of an experiment is the group that receives the experimental treatment (caffeine). In this experiment, the POPULATION A of Daphnia magna, or water fleas are the experimental group because they received 0.5g of caffeine.

On the other hand, the control group is the group that does not receive the experimental treatment. In this experiment, POPULATION B did not receive any caffeine, hence, it is the CONTROL GROUP.

Constant or controlled variable are those variables that the experimenter must keep unchanged for all groups throughout the experiment, in order not to influence the experiment's outcome. In this experiment, the constants are: SAME TEMPERATURE, SAME AMOUNT OF FOOD, SAME TIME BEFORE CHECKING (2hrs)