Why is it not possible to determine causality from a correlation?
a. The correlation proves that there is a causal effect, but you can't know which one causes the other.
b. Correlational studies lack a large enough N, or number of subjects to show causation.
c. There could be a third variable causing both.
d. It is possible, but only if it is a positive correlation.
Answer: C. There could be a third variable causing both.