A difference between Flight 93 and the other planes involved in the September 11th attacks was that

Passengers on Flight 93 learned that their plane had been hijacked and that the hijackers intended to use it in an attack. They organized a revolt, and the plane crashed in Pennsylvania before reaching its target. This distinguished Flight 93 from the other hijacked planes, whose passengers did not mount a similar response. The correct choice is “Passengers became aware of the terrorists’ plans.”

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