How Do Case-Control and Cohort Designs Differ

How do case-control and cohort designs differ?

A) In case-control, the groups are studied prospectively, but in cohort designs, the groups are studied retrospectively
B) In case-control, only one group is studied, but in cohort designs, more than one group is studied
C) In case-control, the groups are selected on the basis of an outcome that has already occurred, but in cohort designs, the groups are selected before the outcome has occurred
D) In case-control, the groups are selected before the outcome has occurred, but in cohort designs, the groups are selected on the basis of an outcome that has already occurred