The Owners of a Health Club Are Interested in Finding

The owners of a health club are interested in finding out which of two brands of aerobic exercise equipment provides a more vigorous workout. They purchase 10 machines of each type and, for five days between 9 A.M. and 12 P.M., they measure the average pulse rate of each person who is working out on one of these 20 machines. This study is an example of

A) An experiment.
B) An observational study.
C) A paired data experiment.
D) A stratified experiment.