Case Control Study or Cohort Study?

C cohort studies are usually paired up with odds ratios, whereas the case control ersary. It would be impossible to do a prospective cohort study on outcomes that are very rare or outcomes that happen a long time after exposure. Only prospective cohort studies can give us relative risks. So magina, often when we talk about randomize control trials, we like they're single blinded or they're double blinded. What does this mean?