A difference between Flight 93 and the other planes involved in the September 11th attacks was that passengers became aware of the terrorists’ plans. Option D is correct.
United Airlines Flight 93, was scheduled to flight from Newark, New Jersey, to San Francisco, California, departing at 8:42 a.m., just minutes before the first hijacked plane struck the World Trade Center.
The passengers and crew of Flight 93 made a series of calls on their cell phones and the in-flight Airfones, informing their families as well as officials on the ground of the plane’s hijacking. As soon as they learned the destiny of the three other hijacked flights in New York City and Washington, D.C., the passengers realized that their plane was involved in a terrorist plot and would be used to carry out further attacks on U.S. soil.