List only posts from primary category

I looked through your help file, but couldn’t find an answer to this. I’d like to know if there is a way to list posts only by the main category? For example, blog post 1 would only show up in a list of posts for Category A even though it’s also in category B. And blog post 2 would only show up in a list for category B.

Blog post 1
Category A – main category
Category B

Blog post 2
Category A
Category B – main category

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