Why Is the Concept of "Age Cohorts" Important in Understanding

Why is the concept of "age cohorts" important in understanding aging?

A) because people born in the same period experience the same historical events at the same time in their life cycle
B) because it standardizes the effects of biology and history on the behaviours observed in the elderly
C) because longitudinal studies cannot control for the effects of cultural changes on behaviours, and therefore nested designs need to be employed
D) because qualitative, rather than quantitative, study designs are more effective in describing inter-cohort differences