Final answer:
On September 11, 2001, planes were hijacked and used as weapons in a series of terrorist attacks, including the World Trade Center in New York City.
Step-by-step explanation:
The subject of this question is Social Studies, as it pertains to the events that occurred on September 11, 2001.
On that day, four planes were hijacked by terrorists. One plane crashed in a field in Pennsylvania, another crashed into the Pentagon, and two planes crashed into the World Trade Center in New York City.
These attacks, known as the 9/11 attacks, had a significant impact on the United States and the world, leading to changes in national security measures and international relations.
Learn more about 9/11 attacks