Which of the Following Statistical Studies Is an Observational Study

Multiple Choice

Which of the following statistical studies is an observational study? ​

A) A group of students is surveyed to determine the percentage who believe in reincarnation.
B) A random sample of students is shown a documentary on the evolution of reincarnation beliefs throughout human history.A second random sample is not shown the documentary.The object of the study is to determine whether viewing the documentary affects belief in reincarnation.
C) To determine whether Vitamin C has any effectiveness at lessening the duration of the common cold, one set of cold sufferers is given no Vitamin C, a second set is given 2 grams per day, and a third set is given 5 grams of Vitamin C per day.
D) All of these studies are observational.
E) None of these studies is observational.