It Has Been Hypothesized That Overall Academic Success for Freshmen

It has been hypothesized that overall academic success for freshmen at college as measured by
Grade point average (GPA) is a function of IQ scores (X1) , hours spent studying each week (X2) , and
One's high school average (X3) . Suppose the regression equation is:
Y' = -6.9 + 0.055X1 + 0.107X2 + 0.0083X3.
The multiple standard error is 6.313 and R2 = 0.826.For which independent variable does a unit
Change have the least effect on GPA?

A) IQ scores (X1)
B) hours spent studying each week (X2)
C) high school average (X3)