One of the Problematic Aspects of the Research into the Link

One of the problematic aspects of the research into the link between stress and illness is that

A) separate control groups are not able to be created due to ethical constraints.
B) the data is correlational in nature and does not allow for conclusions as to causality.
C) subjects are only able to be followed for limited time periods so it is difficult to determine the long-term impact of the stressor.
D) subjective cognitive appraisal differences leads people to interpret stressors differently and interferes with consistent conclusions.