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 are correlational in nature and do 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 lead people to interpret stressors differently and interfere with consistent conclusions.