What Effect Have Televised Presidential Debates Had on Election Outcomes

What effect have televised presidential debates had on election outcomes?

A) The impact of debates is unclear.
B) Research shows that debates are very effective in swaying voter opinion.
C) The majority of the potential voters will change their candidate preference following the debate.
D) Historically the winner of the debates usually wins the presidency.
E) It almost always helps the underdog candidate at the expense of the major candidate.