I'd bet that the answer they were looking for was 100 yards which is considered the "playing filed" if I'm not mistaken.
Reminds me of when I answered a trivia question & was told that was wrong. The question was, "What three animals were in the movie 'The Incredible Journey'. " My answer was a Siamese cat, a bull tarrier, and an Irish setter.
The answer on the card was, "Two dogs and a cat." So I had obviously erred in my answer.