Researchers divided a group of people on the basis of their smoking status (smokers and non-smokers) and followed them for 20 years to see if they developed lung cancer. This is an example of what type of epidemiological research design

Cohort

Explanation:

Cohort studies refer to a form of medical study which is used to examine the reasons for the disease and to find its relation to the risk factors to medical outcomes. Usually, cohort studies track large groups of people, tracking their susceptibility to such risk variables in order to obtain answers to the potential causes of disease.  

As per the question, researchers observe a group of people based on their smoking habits and tracked them for twenty years to observe its relation to lung cancer reflects the procedure of cohort studies.

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answer; elasticity of the lungs decreases;