Why is the finding that "people with mental illness are more likely to smoke" problematic?

A) It was derived from a survey.
B) Correlation does not prove causation.
C) The experiment that reached this conclusion cannot be replicated.
D) This finding is not problematic.

The finding that "people with mental illness are more likely to smoke" is problematic because it only shows a correlation and does not prove a causation. It may be that smoking causes mental illness or that mental illness causes smoking, or there may be other underlying factors that contribute to both. Therefore, further research is needed to understand the relationship between mental illness and smoking.