Context: The 767 has been involved in six hijackings, three resulting in loss of life, for a combined total of 282 occupant fatalities. On November 23, 1996, Ethiopian Airlines Flight 961, a 767-200ER, was hijacked and crash-landed in the Indian Ocean near the Comoros Islands after running out of fuel, killing 125 out of the 175 persons on board; survivors have been rare among instances of land-based aircraft ditching on water. Two 767s were involved in the September 11 attacks on the World Trade Center (1973-2001) in 2001, resulting in the Collapse of the World Trade Center. American Airlines Flight 11, a 767-200ER, crashed into the North Tower, killing all 92 people on board, and United Airlines Flight 175, a 767-200, crashed into the South Tower, with the death of all 65 on board. In addition, more than 2,600 people were killed in the towers or on the ground. A foiled 2001 shoe bomb plot involving an American Airlines 767-300ER resulted in passengers being required to remove their shoes for scanning at US security checkpoints.

Question: How many years had the World Trade center been standing before resulting in collapse?

Answer:
28