Researchers surveyed 959 ninth graders who attended three large U.S. urban high schools and found that those who listened to music that had references to marijuana were almost twice as likely to have used marijuana as those who did not listen to music with references to marijuana (Primack et al. (2010)).

(a) Was this an observational study or an experiment? (b) Can the researchers conclude that listening to music with references to marijuana causes students to use drugs? Explain. (c) Can the researchers extend their results to all urban American adolescents? Explain.