African-American who fight for the union during the Civil War

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Explanation:

In early 1861, General Butler was the first known Union commander to use black contrabands, in a non-combatant role, to do the physical labor duties, after he refused to return escaped slaves, at Fort Monroe, Virginia, who came to him for asylum from their masters, who sought to capture and reenslave them.

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option d is correct. corroborated sources of historical evidence   is necessary to validate a claim about an event in history.

historians tend to use written sources, but audio and visual materials as well as artifacts are important objects that supply information to modern historians.

historical sources are usually divided into two categories: primary and secondary sources. depending on the historian's intent, some sources shift their designation. determining what type of sources toimplement, and the level of credibility and reliability of those sources, is a significant step in critical thinking for the historian.

explanation: