The difference between an observational study and an experiment is that
A
in an observational study, only one group is studied, and in an experiment, two groups are studied.B
in an observational study, the researchers do not control treatment, and in an experiment, they do.C
in an experiment, cause-and-effect is analyzed, and in an observational study, it is not.D
in an experiment, one group is studied over a short period of time, and in an observational study, the group is studied over a longer period of time.