What Is One of the Main Problems in Determining Whether

What is one of the main problems in determining whether one's depression is endogenous or reactive?

A) Most people forget,or repress,the stressful events that may have led to their depression.
B) It is unclear whether exposure to a stressor was a contributing factor or if it was a coincidence.
C) People with depression often underreport being exposed to stressful events.
D) It is virtually impossible to study endogenous factors that are linked to depression.