In a Study on Treatments for Anorexia, Researchers Recruited 30

In a study on treatments for anorexia, researchers recruited 30 teenage girls with anorexia. The girls were ranked by weight prior to the study. Of the heaviest three girls, one was randomly assigned to a "family" therapy, one to a "cognitive/behavioral" therapy, and one to a control group. The same was done for the next three heaviest girls, down to the three lightest girls. After five weeks of treatment, researchers weighed the participants. Researchers would like to determine which, if any, of the treatments are most effective in treating anorexia.
-Can the results from this study be generalized to the population of all teenage girls with anorexia?

A) No, since the participants in the study were volunteers.
B) No, since it is an observational study.
C) Yes, since it is a randomized experiment.
D) Yes, since the treatments were randomly assigned.