A study of treatments for angina (pain due to low blood supply to the heart) compared bypass surgery, angioplasty, and use of drugs. The study looked at the medical records of thousands of angina patients whose doctors had chosen one of these treatments. It found that the average survival time of patients given drugs was the highest. What do you conclude?

(a) This study proves that drugs prolong life and should be the treatment of choice.

(b) We can conclude that drugs prolong life because the study was a comparative experiment.

(c) We can’t conclude that drugs prolong life because the patients were volunteers.

(d) We can’t conclude that drugs prolong life because this was an observational study.

(e) We can’t conclude that drugs prolong life because no placebo was used.