Question: In designing a cohort study, which of the following characterizes the concept of exchangeability? a. We enroll a counterfactual group that is as similar as possible to the individuals in the exposed group. b. We enroll an unexposed group that is as similar as possible to the individuals in the control (vs intervention) arm. c. We enroll an unexposed group that is as similar as possible to the counterfactual state of the exposed group. d. We select controls at the same time their matched cases are diagnosed