Which of the following control variables would be more likely to cause bias due to being a mediating factor for estimating the effect of class size on 6th grade student math achievement? Explain....

Which of the following control variables would be more likely to cause bias due to being a mediating factor for estimating the effect of class size on 6th grade student math achievement? Explain. Control variable 1: The amount of parental help on homework. Control variable 2: The amount of individual time with the teacher