Ans. is 'b' i.e., Beta carotene and lung cancer have no relation to each other Prospective (coho) study is used to test the association between risk factor and disease. o Relative risk (estimated by coho study) is a direct measure of the strength of association between suspected cause and effect. o Relative risk one indicates no association between risk factor and outcome. o Relative risk more than one indicates positive association --> risk factor is causative. o Relative risk less than one indicates negative association --> that means exposure has protective role on outcome, e.g. vitamin A exposure results in decreased incidence of skin cancer. o In this question, RR is one. That means beta carotene exposure is neither causative nor protective for lung cancer.