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.
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.