Why Are Swing States So Important to a Campaign Strategy

Why are swing states so important to a campaign strategy?

A) They obviously favor one party's candidate over another, which saves the favored candidate money to campaign elsewhere.
B) They provide better opportunities for fund-raising.
C) They can be ignored.
D) They can potentially be won by either major party candidate.
E) They are always small states, so they do not provide that many electoral college votes.