In each case, determine whether the study in question is a controlled experiment or an observational study; justify your answer.
(a) Studying whether drivers who pass through a certain intersection signal their turns at that intersection.
(b) Tracking the blood pressure of patients in two groups; the members of one have been given a certain drug by the investigators, and the members of the other have not.
(c) An investigator compares the incidence of crime in New York and New Jersey.