Question: For each of the following, decide whether it is an observational study or a randomized experiment?

(a) A group of 100 students was randomly divided, with 50 assigned to receive vitamin C, and the remaining 50 to receive a placebo, to see if vitamin C helps prevent colds.

(b) All patients who underwent hip replacement surgery at Stanford University Hospital during 1990 to 2000 will be followed for 10 years after their operation to determine the success (or failure) of the replacement.

(c) A group of students who were enrolled in an Introductory Statistics course were randomly assigned to take a Web-based course or to take a traditional lecture course. The two methods were compared by giving the same final examinations in both courses.

(d) A group of smokers and another group of nonsmokers who visited a particular clinic were asked to come in for a physical exam every 5 years for the rest of their lives in order to monitor and compare their health status.