Which statement is a main idea of The Mary Celeste: An Unsolved Mystery from History?

As an "open file" – a mystery still waiting to be solved.

Explanation: In this story, we learn that the "Mary Celeste" is considered an "open file" by the father, as it is a mystery that is waiting to be solved. The Mary Celeste was a ship that was found 400 miles east of the Azores on December 5, 1872. It was found by the crew members of the British brig Dei Gratia. The Mary Celeste had left New York City eight days earlier and should have safely arrived in Genoa, Italy. However, the ship was discovered to be empty, with no sign of damage. No one knows what happened to the crew, which makes this a mystery still waiting to be solved.

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lee and grant, both of whom held the highest rank in their respective armies, had known each other slightly during the mexican-american war (1846-48) and began their dialogue by exchanging awkward personal inquiries. characteristically, grant had arrived in his mud-splattered field uniform while lee had turned out in full dress attire, complete with sash and sword.

explanation:

these civilizations were located around fertile rivers, where they could produce an abundance of crops