In the passage, the factor supporting the two-party system is winner-takes-all elections (C).
In the United States, 48 states and the District of Columbia have the winner-takes-all rule for the Electoral College. In these states, whichever canidate recieves a majority of the votes or the candidate that has less than 50% of the votes, but still more than any other candidate, takes all of the Electoral votes. This means that if someone got 10% of the votes, they wouldn't be represented in the Electoral College.