Identify each study as being either observational or experimental.
a. Subjects were randomly assigned to two groups, and one group was given an herb and the other group a placebo. After 6 months, the numbers of respiratory tract infections each group had were compared.
b. A researcher stood at a busy intersection to see if the color of the automobile that a person drives is related to running red lights.
c. A researcher finds that people who are more hostile have higher total cholesterol levels than those who are less hostile.
d. Subjects are randomly assigned to four groups. Each group is placed on one of four special diets—a low-fat diet, a high-fish diet, a combination of low-fat diet and high-fish diet, and a regular diet. After 6 months, the blood pressures of the groups are compared to see if diet has any effect on blood pressure.