Cosella is conducting an experiment where she assesses how quickly teenagers can run a 100-meter race after consuming specific amounts of caffeine. She divides her sample up into three groups. Group 1 receives a glass of water with no caffeine added. Group 2 receives a glass of water with an amount of caffeine equivalent to that in one cup of coffee. Group 3 receives a glass of water with an amount of caffeine equivalent to that in two cups of coffee. Each participant is then timed as they run the course. In this study, the independent variable is

a. the previous running experience of each participant
b. the teenagers who are being studied
c. the time it takes to run the 100-meter race
d. the amount of caffeine being ingested

An independent variable is the variable which is not being controlled and it does not depend on the other variables. This is the variable which is being studied/measured in the experiment.

Option a. The previous running experience is not being considered and is not a variable under study in this case. So this is not the answer.

Option b. The teenagers who are being studied constitute the sample. These are not the variables.

Option c. Time is the independent variable, as it is being measured during the experiment and the conclusion is being drawn based on it.

Option d. Amount of caffeine is being decided by Cosella and is therefore not the independent variable.

Therefore, the correct answer is option c