Part A
The main idea of The Mary Celeste: An Unsolved Mystery from History is:
One theory about the fate of the people on board the Mary Celeste is that they disappeared on purpose.
Part B
The detail from the text that best supports the answer to Part A is:
“The only people who knew what really happened on the Mary Celeste after the last log entry had been written were the ten people on board.”
This detail emphasizes the mystery surrounding the disappearance of the crew and supports the idea that their fate remains uncertain, aligning with the theory that they might have disappeared on purpose.