whNULL aliweka alama kwenye shoka kwa weirdlNULL? WhNULL alitumia z kwa shoka wima wakati angeweza tu kutumia NULL ili kuepuka kuchanganyikiwa?####[['NULL', 'Instructor', 'NEG'], ['shoka', 'Teaching_Setup', 'NEG'], ['z', 'Mathematical_Related_Concept', 'NEU'], ['shoka wima', 'Mathematical_Related_Concept', 'NEU'], ['y', 'Mathematical_Related_Concept', 'NEU']]