A Democrat can vote in a Republican primary in states that hold open primaries. hold closed primaries. have indirect primaries. have party caucuses.

Hold open primaries

Explanation:

In an open primary, anyone can vote, regardless of their party affiliation. In a closed primary, only a republican voter can vote in the republican primary. In an indirect primary, voters choose delegates who will vote as theri representative. A caucuse isn't so much an election as a planning meeting.

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