When Does Confounding Occur

When does confounding occur?

A) When the characteristics of the participants in the exposed group differ in some important way from those in the unexposed group
B) When the characteristics of the participants in the study sample differ in some important way from those in the source population
C) When the controls are not matched to the cases in a case-control study
D) When the people lost to follow‐up in a cohort study differ from those who are not lost to follow‐up
E) It occurs in all epidemiological research