Question: List all the explanations about object classes of all the images with an x and y coordinate of 0.
SQL: SELECT T1.OBJ_CLASS FROM OBJ_CLASSES AS T1 INNER JOIN IMG_OBJ AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID WHERE T2.X = 0 AND T2.Y = 0 GROUP BY T1.OBJ_CLASS