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.

(b) in an observational study, the researchers do not control treatment, and in experiment, they do.

Step-by-step explanation: In an observational study, only one group is studied, but in an experiment, more than one, may be two, three, etc. are studied, so option (A) is not correct. In an observational study, researchers control treatment but in experiment they do not, and so, option (B) is also incorrect. Option (D) is not correct, because a group is studied over a longer period of time in an experiment and the group is studied over a short period of time in observational study. The only correct option is (C) because cause and effect is studied in an experiment and are not studied in an observational study.

Hence, (C) is the correct option.