Ryan has constructed a multiple regression model and wants to test some assumptions of his regression model. In particular, he is testing for normality of the residuals. To do this Ryan should: a. construct a normal quantile plot of the residuals and see if the result is an approximately upward sloping straight line
b. plot the residuals vs the predicted values of the response variable and check if the result is a random pattern
c. plot the residuals vs the observed values of the response variable and check if it forms and upward sloping line
d. plot the residuals vs each explanatory variable to check if they are approximately random and form no clear pattern
e. construct a normal quantile plot of the residuals and see if the result is an approximately horizontal straight line
f. construct a normal quantile plot of the residuals and see if the result is an approximately downward sloping straight line