Answer: D. It can be conducted with existing data
Step-by-step explanation:
Cohort studies observe a number of people over a period of time to discover the factors pertaining to a disease such as its causes and effects. It can be prospective, which uses data gathered in the future or retrospective which uses data already collected.
Prospective cohort studies are useful in estimating risk and impractical for studying rare diseases due to the need for a large group. When used to identify the cause of a disease, a hypothesis is tested by monitoring a cohort for any differences in health associated with the risk factor .
Existing data. and the study of diseases with a long latency between exposure and symptoms development apply to retrospective cohort study.