How are the republican and democratic presidential candidates chosen

The delegates to the national convention will vote, by ballot, to select the Republican Party's nominee for President of the United States in the election, where the majority will be bound by the results of their respective state contests on the first ballot.                                        

The party's presidential nominee is chosen primarily by pledged delegates, which are in turn selected through a series of individual state caucuses and primary elections. Superdelegates, delegates whose votes are not bound to the outcome of a state's caucus or primary, may also influence the nomination.

Explanation:

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i would say b. research typically uses a variety of sources

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