19. A researcher tests whether epicatechin, an antioxidant found in green tea and dark chocolate, can help people recover from a workout. Of the 40 volunteers, 20 are randomly assigned to receive the antioxidant while 20 receive a placebo. A technician, unaware of which group each volunteer was in, measures F2-isoprostanes to assess the oxidative stress level of the subjects after exercise on a treadmill. What type of design was used in this study? A. Simple comparative experiment B. Block design experiment C. Matched pairs experiment D. None of the above; this is an observational study