A recent study in Denmark between 1994 and 2012 evaluated the risk of death and hospitalization due to chronic obstructive pulmonary disease (COPD). The study found that in Denmark, birth years between 1925 and 1930 were associated with higher risk of severe COPD than birth years before or after. The observed variation in COPD risk among the cohort of individuals born in birth years 1925-30 is an example of which type key concept in life course epidemiologic (choose one best answer)?

cohort effect

mediation

accumulation of risks

a trigger effect within a pathway

none of the above