Calculate the average of authors for each paper from the year of 1990 to 2000.
Average refer to DIVIDE(COUNT(AuthorID where Year = 1990 BETWEEN 2000), COUNT(Title where Year = 1990 BETWEEN 2000))
SELECT CAST(COUNT(DISTINCT T2.AuthorId) AS REAL) / COUNT(DISTINCT T1.Title) FROM Paper AS T1 INNER JOIN PaperAuthor AS T2 ON T1.Id = T2.PaperId WHERE T1.Year BETWEEN 1990 AND 2000
database name:db_id authors
Author table Author Cols: Id dtype integer, Name dtype text, Affiliation dtype text, 
Conference table Conference Cols: Id dtype integer, ShortName dtype text, FullName dtype text, HomePage dtype text, 
Journal table Journal Cols: Id dtype integer, ShortName dtype text, FullName dtype text, HomePage dtype text, 
Paper table Paper Cols: Id dtype integer, Title dtype text, Year dtype integer, ConferenceId dtype integer, JournalId dtype integer, Keyword dtype text, 
PaperAuthor table PaperAuthor Cols: PaperId dtype integer, AuthorId dtype integer, Name dtype text, Affiliation dtype text, 
