A Researcher Discussing the Prevalence of Eating Disorders Makes Reference

A researcher discussing the prevalence of eating disorders makes reference to what she says is a cohort effect for these disorders.What does this mean?

A) Eating disorders tend to run in close-knit families.
B) Women born after 1960 have a higher rate of bulimia nervosa than women born prior to 1960.
C) Families that place a special emphasis on eating meals together have high rates of eating disorders.
D) The frequency of eating disorders is strongly related to the rate among friends,especially in high school.