US District Courts serve as the “finders of fact”. What does this mean?

Finder of fact, is a person, or group of persons, who determines facts in a legal proceeding, usually a trial. To determine a fact is to decide, from the evidence, whether something existed or some event occurred.[1] Various aspects of a case that are not in controversy may be the "facts of the case" and are determined by the agreement of the separate parties; the trier of fact need not decide such issues.

Yes, the correct answer is b.

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