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.