What is the difference between an observational study and an​ experiment? Choose the correct answer below. A. 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. B. 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. C. 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. D. 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.