Concurrent powers have which of the following effects on the united states political system? a. they reinforce the expansion of the power of the national government and the supremacy of the national law. b. they eliminate disputes between the states and the national government by creating completely separate spheres of influence. c. they foster cooperation between the states and the national government by requiring that the two levels of government work together. d. they make it possible for the states and the national government to simultaneously exercise influence in the same areas of public policy.