In Addition to Gerrymandering, a Major Reason Why So Many

In addition to gerrymandering, a major reason why so many members of Congress are elected from "safe" districts is

A) that local newspapers and television stations typically provide very favorable coverage to the incumbent party's candidates.
B) the natural geographic clustering of people who share a political ideology.
C) that once people move into a district that is very liberal or very conservative, they tend to switch their ideology to match that of their new districts.
D) that candidates for Congress usually change their partisan affiliation to match that of their districts.