What is the average height in centimeters of all the players in the position of defense?
average = AVG(height_in_cm); players refers to PlayerName; position of defense refers to position_info = 'D' ;
SELECT CAST(SUM(T2.height_in_cm) AS REAL) / COUNT(T1.ELITEID) FROM PlayerInfo AS T1 INNER JOIN height_info AS T2 ON T1.height = T2.height_id WHERE T1.position_info = 'D'