Which of the following is a key distinction between well designed experiments and observational studies?
(A) More subjects are available for experiments than for observational studies.
(B) Ethical constraints prevent large-scale observational studies.
(C) Experiments are less costly to conduct than observational studies.
(D) An experiment can show a direct cause-and-effect relationship, whereas an observational study cannot.
(E) Tests of significance cannot be used on data collected from an observational study
