A researcher has found a strong relationship between level of education and income and concludes that "higher levels of education cause higher income." What error has been made?
a. None, the argument is sound.
b. Income is only nominal in level of measurement and education is ordinal so they cannot be causally related.
c. Since these variables are continuous and interval ratio, they cannot have a causal relationship.
d. By itself, association does not prove causation.