Name the object class of the image with lowest bounding box.
bounding box refers to X, Y, W, H from IMG_OBJ; lowest relates to the height of the bounding box which refers to MIN(H);
SELECT T2.OBJ_CLASS FROM IMG_OBJ AS T1 INNER JOIN OBJ_CLASSES AS T2 ON T1.OBJ_CLASS_ID = T2.OBJ_CLASS_ID ORDER BY T1.H LIMIT 1