List all the attribute classes of the image ID "15".
attribute classes of the image ID "15" refer to ATT_CLASS where IMG_ID = 15;
SELECT T1.ATT_CLASS FROM ATT_CLASSES AS T1 INNER JOIN IMG_OBJ_ATT AS T2 ON T1.ATT_CLASS_ID = T2.ATT_CLASS_ID WHERE T2.IMG_ID = 15