List all the explanations about object classes of all the images with an x and y coordinate of 0.
explanations about distinct object classes refers to OBJ_CLASS; images refers to IMG_ID; x and y coordinate of 0 refers to X = 0 AND Y = 0
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