A study attempts to compare two sunscreens. Each of 50 subjects with varying skin complexions will employ both sunscreens - Screen A on one side of his or her body and Screen B on the other side. For each subject, a coin is tossed in order to determine which side receives Screen A and which receives Screen B. Researchers measure the amount of ultraviolet light exposure over both treated areas for each subject. This is an example of
A. a matched pairs experiment.
B. a double-blind observational study.
C. a stratified analysis.
D. the placebo effect.