Or it could be argued that the answer is two. The north tower and the south tower were distinct collections of glass and steel separated by an expanse of space, and they were hit at different times and went out of existence at different times. The amateur video that showed the second plane closing in on the south tower as the north tower billowed with smoke makes the twoness unmistakable: in those horrifying moments, one event was frozen in the past, the other loomed in the future.
