A Strong Correlation Exists Between Stressful Life Events and the Onset

A strong correlation exists between stressful life events and the onset of depression,but it is difficult to interpret this relationship because

A) there haven't been enough studies.
B) stress can cause depression,but depression can cause stress.
C) depression is a very subjective state and thus very difficult to measure.
D) people are usually aware of any connection between their depression and stress in their lives.