In order to determine the effect companion animals might have on the moods of elderly people living alone experimenters created six groups from a large population of single people over 70 years old. They had three groups: those without pets, those who owned a dog, and those who owned a cat. They randomly selected 50 men from each of those groups and 50 women from each of those groups for a total of 300 participants. Which of the following statements about this study is correct?

a. This study is an experiment, since it was based on a random sample.
b. This study is an observational study, since researchers did not assign the subjects to a gender or pet ownership status.
c. This study is an experiment, since each subject was classified into one of the six possible combinations of pet ownership and gender.
d. This study is a combination of both an experiment and an observational study.