How Has Party Loyalty in the U

How has party loyalty in the U.S.Congress changed since the early 1970s?

A) It varies with the personality and media tactics of the president.
B) It has trended downward for the last three decades.
C) It has trended upward for the last three decades.
D) It has remained essentially the same, with occasional shocks up and down.
E) Southern Democrats have become more conservative, while Northeastern Republicans have become more liberal.