Polly Wants to Determine If the Type of High School

Polly wants to determine if the type of high school one attends affects his or her performance in college. Assuming Polly's socioeconomic status impacts her college choices, there may be a third variable influencing the results. In this example, Polly's socioeconomic status is what type of variable?

A) independent variable
B) dependent variable
C) control variable
D) intervening variable