Given the task definition and input, reply with output. In this task, you're given passages that contain mentions of names of people, places, or things. Some of these mentions refer to the same person, place, or thing. Your job is to write questions that evaluate one's understanding of such references. Good questions are expected to link pronouns (she, her, him, his, their, etc.) or other mentions to people, places, or things to which they may refer. Do not ask questions that can be answered correctly without understanding the paragraph or having multiple answers. Avoid questions that do not link phrases referring to the same entity. For each of your questions, the answer should be one or more phrases in the paragraph, and it should be unambiguous.

Passage: The concert took place on Saturday, September 19, 1981, on the Great Lawn, the central open space of Central Park. The first spectators, many carrying chairs or picnic blankets, arrived at daybreak to secure a good spot. The Parks Department originally expected about 300,000 attendees. Although rain fell throughout the day and continued until the start of the concert, an estimated 500,000 audience members made this the seventh-largest concert attendance in the United States in history.The stage backdrop depicted an urban rooftop with water tank and air outlet, symbolic of New York's skyline.  At twilight, the backing band went onstage, followed by New York's mayor, Ed Koch, who announced, "Ladies and gentlemen, Simon and Garfunkel!" The duo entered through a side stage door, took center stage amid audience applause, looked at each other and shook hands, and began the concert with their 1968 hit "Mrs. Robinson".
After the second song, "Homeward Bound", Simon delivered a short speech which began, "Well, it's great to do a neighborhood concert." He then thanked the police, the fire department, the park administration and finally Ed Koch. The audience booed at the mention of Koch but applauded as Simon continued.
Simon & Garfunkel played twenty-one songs in total: ten by the duo, eight by Simon, one by Garfunkel, a cover of  The Everly Brothers' "Wake Up Little Susie", and the medley version of "Maybellene". Each performer sang three songs alone, including one new song apiece. Garfunkel sang the Simon & Garfunkel classic "Bridge Over Troubled Water" and "April Come She Will", and "A Heart in New York", a song written by Gallagher and Lyle that appeared on his album Scissors Cut, which had been released the previous month. Simon's solo performances were the title song of his 1975 album Still Crazy After All These Years, the number-one single "50 Ways to Leave Your Lover", and the unreleased "The Late Great Johnny Ace", which would appear on his 1983 album Hearts and Bones.
Whose concert was the seventh-largest in United States history?