In the US,One's SAT Score May Be Used Is a "Proxy" Measure

In the U.S.,one's SAT score may be used is a "proxy" measure of one's social class.What does this mean?

A) The SAT score is a more realistic measure of social class than income or education.
B) SAT scores are the best predictors of how well a student will do in college.
C) Family income is strongly correlated with scores on the SAT exam.
D) SAT scores are more related to race than income.