A number of competitive female cross country runners suffer from bone loss due to the low

A number of competitive female cross country runners suffer from bone loss due to the low estrogen levels. Some medical experts conjecture that this can be prevented by taking oral contraceptives, as those contain estrogen. This conjecture is to be tested with an experiment. The goal of the experiment is to find out whether taking an oral contraceptive prevents bone loss in female cross country runners. Which of the following subjects should be recruited in order to do a good experiment? (Pick one of the three.)