The collapse of the World Trade Center Twin Towers was the first of its kind for steel structures. There were multiple unique designs implemented in these towers. Out of all these designs, what was the main factor that contributed to the collapse (i.e. if it was avoided the towers will still be standing until today).

a. All columns were relocated to the perimeter of the towers.

b. The flooring trusses were connected to the columns using two bolts on each side. c. Replaced concrete walls protecting the core with sheetrock walls.

d. All evacuation staircases were located at the center of the towers (i.e. near the core)