To test whether extensive exercise lowers the resting heart rate, a study is performed by randomly selecting half of a group of volunteers to exercise 1 hour each morning, while the rest are instructed to perform no exercise. Is this study an experiment or an observational study?
A. An experiment with a control group and blinding
B. An experiment with blocking
C. An observational study with comparison and randomization
D. An observational study with little if any bias
E. None of the above