In 1975, what was the average point of all-star players coming from the USA?
in 1975 refers to season_id = 1975; coming from USA refers to birthCountry = 'USA'; average point refers to Divide (Sum(points), Count(playerID))
SELECT AVG(T2.points) FROM players AS T1 INNER JOIN player_allstar AS T2 ON T1.playerID = T2.playerID WHERE T2.season_id = 1975