Which of the following is the primary difference between an experiment and an observational study?
(A)Experiments are only conducted on human subjects; observational studies can be conducted on nonhuman subjects. (B)In an experiment, the researcher manipulates some variable to observe its effect on a response variable; in an observational study, he or she simply observes and records the observations. (C)Experiments must use randomized treatment and control groups; observational studies also use treatment and control groups, but they do not need to be randomized. (D)Experiments must be double-blind; observational studies do not need to be.
A