Cosella Is Conducting an Experiment Where She Assesses How Quickly

Multiple Choice

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.