What is the difference between an observational study and an experiment?

Question
Choose the correct answer below.

A.
In an experiment, a treatment is applied to part of a population and responses are observed. In an observational study, a researcher measures characteristics of interest of a part of a population but does not change existing conditions.

B.
In an experiment, a researcher measures characteristics of interest of an entire population but does not change existing conditions. In an observational study, a treatment is applied to an entire population and responses are observed.

C.
In an experiment, a researcher measures characteristics of interest of a part of a population but does not change existing conditions. In an observational study, a treatment is applied to part of a population and responses are observed.

D.
In an experiment, a treatment is applied to an entire population and responses are observed. In an observational study, a researcher measures characteristics of interest of an entire population but does not change existing conditions.