One Proposed Explanation for Why Congressional Seats Have Become Less

 One proposed explanation for why congressional seats have become less marginal is that 

A)  the growing strength of the party system makes it less likely for challengers to appeal to average voters. 
B)  incumbents can use their powers to get programs passed that benefit their districts and thereby themselves. 
C)  incumbents are less likely than challengers to become associated with the "mess" in Washington. 
D)  changing demographics have made an increasing number of districts overwhelmingly Democratic or Republican. 
E)  incumbents are less ideological than challengers and link themselves with Congress as an institution.