Echinacea has been widely used as an herbal remedy for the common cold, but previous studies evaluating its efficacy as a remedy have produced conflicting results. In a new study, researchers randomly assigned 437 volunteers to receive either a placebo or echinacea treatment before being infected with rhinovirus. Healthy young adult volunteers were recruited for the study from the University of Virginia community.

(a) Identify the population of interest and the sample in this study.

(b) Comment on whether or not the results of the study can be generalized to a larger population.

(c) Can the findings of the study be used to establish causal relationships? Justify your answer.