Among the Most improved Players awarded from 1985-1990, how many player whose country is USA?
the Most improved Player refers to award = 'Most Improved Player'; from 1985-1990 refers to year between 1985 and 1990; country is USA refers to birthCountry = 'USA'
SELECT COUNT(DISTINCT T2.playerID) FROM awards_players AS T1 INNER JOIN players AS T2 ON T1.playerID = T2.playerID WHERE T1.award = 'Most Improved Player' AND T2.birthCountry = 'USA' AND T1.year BETWEEN 1985 AND 1990