S1 문장의 [______]에 들어갈 단어는?

난이도 고난도 · 힌트를 덜 열수록 높은 점수 (현재 5점)
As the tenth [______] of the terrorist attacks of September 11, 2001, approached, 9/11 related media stories peaked in the days immediately surrounding the anniversary date and then dropped off rapidly in the weeks thereafter.