You have estimated a linear regression model relating y to x. your professor says, "i think that the relationship between y and x is nonlinear." how would you test the adequacy of your linear regression? (check all that apply )
a. compare the fit between of linear regression to the non-linear regression model.
b. there is evidence in favor of a non-linear relationship if there is zero correlation between the dependent and independent variable.
c. if adding a quadratic term, you could test the hypothesis that the estimated coefficient of the quadratic term is significantly different from zero.
d. all of the above.