What Did Political Scientist Larry Bartels Demonstrate in His Book

What did political scientist Larry Bartels demonstrate in his book Unequal Democracy?

A) Elected officials are substantially more responsive to the concerns of their more affluent constituents than to those of their poorer constituents.
B) Elected officials are substantially more responsive to the efforts of lobbyists than to the concerns of their constituents.
C) Elected officials do not tend to respond to the efforts of social movements or protests when casting votes or forming policy.
D) Elected officials tend to ignore the interests of their constituents when they make policy or cast votes,until just before they are up for reelection.
E) Incumbents hold so much power in the United States and are so difficult to unseat that they have disenfranchised much of the voting population of the U.S.