case-control studies

In case-control studies, the odds ratio is used as an estimate of the relative risk. In order for this approximation to be reasonable, some conditions must be met. Which of the following conditions is not necessary in order to use the odds ratio to estimate the relative risk?

With respect to exposure, controls are representative of the   population to which you want to generalize your results.

The event (disease) under study is rare in the population.

The exposure in question is rare in the population.

Cases are representative of all cases.