Which would be considered a secondary source about the assassination of Abraham Lincoln? a diary entry by a witness to the assassination
an interview with a member of the audience at Ford’s Theater where Lincoln was shot
an interview with Booth’s understudy, who was not at Ford’s Theater the night of the assassination
a newspaper article written by a reporter who was in the audience at Ford’s Theatre

An interview with Booth's understudy

Explanation:

Secondary source is an article or paper talking about a thing, Without seeing it happen. For example, You have someone who saw Abraham Lincoln get shot. Then they write down what happened. The thing they wrote down is a primary source. Your friend tells you what happened in Ford's Theater, but he wasn't actually there. Thats a secondary source. Hope this helps, I spend 15 minutes typing this :)

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it's lower nubia : ) good luck