Why Is It Not Possible to Show Causal Links Between

Why is it not possible to show causal links between stress and illness on the basis of retrospective studies investigating whether people who have recently been ill have experienced stressful life events in the months preceding the illness?

A) After experiencing serious illness people may have a tendency to forget preceding stressful events.
B) It is difficult to be sure whether any particular life event was a stressor causing the illness.
C) Individuals differ in how stressful they find any particular life event.
D) People who have been ill are predisposed to remember recent stressful events to a greater extent than people who have not been ill.