A vertical long-run Phillips curve is consistent with a. both the conclusion of Friedman and Phelps and the classical idea of monetary neutrality. b. neither the conclusion of Friedman and Phelps nor the classical idea of monetary neutrality. c. the classical idea of monetary neutrality, but it is not consistent with the conclusion of Friedman and Phelps. d. the conclusion of Friedman and Phelps, but it is not consistent with the classical idea of monetary neutrality.