Use the Following Information for Questions

Use the following information for questions:
Having a coffee fix just before a workout may not be the best idea, a study suggests. Researchers in Switzerland found that the amount of caffeine in just two cups of coffee limits the body's ability to increase blood flow to the heart during exercise. The study included 18 young, healthy people who were regular coffee drinkers. They did not drink any coffee for 36 hours prior to study testing. The researchers used high-tech PET scans to measure the participants' heart blood flow before and after they rode a stationary bike and then repeated the testing procedure after swallowing a tablet containing 200 milligrams of caffeine -- the amount contained in two cups of coffee. The caffeine did not affect heart blood flow when the participants were inactive. However, measurements taken immediately after exercise showed a slowdown in heart blood flow after they'd taken the caffeine tablets, compared to their previous results.
-Which variable is an explanatory variable in this study?

A) The health of the participants
B) Taking a caffeine tablet (yes or no)
C) Difference in heart blood flow
D) The PET scan