Advertisements for a new energy drink claim that it helps people concentrate, and you are designing an experiment to test the hypothesis that the energy drink improves concentration. A large group of study participants are going to come to your lab and take a concentration test. Now you must choose independent and dependent variables for your experiment. What would be an appropriate independent (treatment) variable for your experiment?
A) How much caffeine (from all sources) participants have consumed the day of the test
B) How much of the energy drink participants report drinking in the last month
C) How well the participants can concentrate during the test
D) How much of the energy drink participants are instructed to drink right before the test