Regarding analysis of residuals, decide in each case which assumption for regression inferences may be violated.
a. A residual plot—that is, a plot of the residuals against the observed values of the predictor variable—shows curvature.
b. A residual plot becomes wider with increasing values of the predictor variable.
c. A normal probability plot of the residuals shows extreme curvature.
d. A normal probability plot of the residuals shows outliers but is otherwise roughly linear.