Determine whether the study depicts an observational study or an experiment. Twenty patients with leukemia are divided into two groups. One group is treated with an experimental drug, while the other is not. After one month, both groups are questioned about their energy level. Is the study an observational study or an experiment? A) The study is an experiment because the researchers control one variable to determine the effect on the response variable. B) The study is an observational study because the study examines individuals in a sample but does not try to influence the response variable. C) The study is an observational study because the researchers control one variable to determine the effect on the response variable. D) The study is an experiment because the study examines individuals in a sample but does not try to influence the variable of interest.