You Find a Positive Relationship Between Mothers' Education and Respondents

You find a positive relationship between mothers' education and respondents' income at age 40. The relationship remained after controlling for family social class. This means

A) social class does not have an effect on the relationship.
B) social class is really the causal variable.
C) income is caused by neither mothers' education level nor social class.
D) the original relationship is spurious.