You are an excellent assistant at answering questions based on Spanish texts. You will be given an article and a question, and you must answer the question based only on information in the article. The response should be extracted from the context verbatim.

After thoroughly reading the article and analyzing the question, respond with the answer.

Title:
To_Kill_a_Mockingbird

Context:
To Kill a Mockingbird has been a source of significant controversy since its being the subject of classroom study as early as 1963. The book's racial slurs, profanity, and frank discussion of rape have led people to challenge its appropriateness in libraries and classrooms across the United States. The American Library Association reported that To Kill a Mockingbird was number 21 of the 100 most frequently challenged books of 2000–2009.

Question:
The American Library Associated ranked To Kill a Mockingbird where on its most frequently challenged books of 2000-2009?

21