What are the attributes of the widest object in image 400?
attributes refer to ATT_CLASS; the widest relates to the width of the bounding
box of the object which refers to MAX(W); image 400 refers to IMG_ID = 400;
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 INNER JOIN IMG_OBJ AS T3 ON T2.IMG_ID = T3.IMG_ID WHERE T2.IMG_ID = 400 ORDER BY T3.W DESC LIMIT 1