9.1. For the following variables in a regression analysis, which variable more naturally plays the role ofx(explana- tory variable) and which plays the role ofy(response vari- able)?

(a) College grade point average (GPA) and high school GPA.

(b) Number of children and mother's education level.

(c) Annual income and number of years of education.

(d) Annual income of homeowner and assessed value of home.