A researcher is studying the impact of caffeine on length of stay after surgery. She enrolls patients who are undergoing an outpatient surgery and, after obtainingconsent, she randomizes patients to either receive a caffeinated drink or a non-caffeinated drink upon waking after surgery. She records the length of stay in the PACU for each patient. What is the independent variable?

Group of answer choices

Surgical patients

Outpatient surgeries only

Caffeinated vs non-caffeinated drink

Length of stay in the PACU