List all the object classes in image 10.
object classes refer to OBJ_CLASS; image 10 refers to IMG_ID = 10;
SELECT DISTINCT T2.OBJ_CLASS FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T1.IMG_ID = 10