This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Unknown
Unit of ecological study is: 
Select and explain. 
A. Population, B. Individual, C. Healthy people, D. Patient
A: Population
Answer: A: Population 
Explanation: Ref. Foundations of Community medicine. Page. 107
 
In ecological studies the unit of observation is the population or community.

Disease rates and exposures are measured in each of a series of populations and their relation is examined.
Often the information about disease and exposure is abstracted from published statistics and therefore does not require expensive or time