How does the presidential primary system contribute to the intense partisanship of American politics today?

a. It allows minor parties to compete against the major parties, forcing the main parties into extremism.
b. It takes power out of the political parties and puts it into the hands of the candidates, who react by taking their positions to the extreme.
c. The conventions create an artificial emotionalism that appeals to extreme elements.
d. It forces candidates to play to the most ideologically extreme elements within their party.

d