Which of the Following Statements About Political Advertising Is LEAST

Which of the following statements about political advertising is LEAST accurate?

A) Some western democracies ban paid advertising and instead give free airtime to candidates.
B) Attack ads seem to have an immediate effect on potential voters.
C) The long-term impact of negative advertising is a matter of dispute.
D) The effectiveness of advertising, positive and negative, has been clearly established and quantified.
E) One form of negative ad is the comparative message, which contrasts each candidate's position on a given issue.