Question: when were weight classes introduced to the ufc
[{'answer': 'at UFC 12', 'provenance': [{'wikipedia_id': '6286355', 'title': 'Mixed martial arts weight classes', 'start_paragraph_id': 3, 'start_character': 224, 'end_paragraph_id': 3, 'end_character': 234, 'bleu_score': 1.0, 'section': 'Section::::Unified Rules of Mixed Martial Arts.\n'}]}, {'provenance': [{'wikipedia_id': '6286355', 'title': 'Mixed martial arts weight classes', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 353, 'bleu_score': 0.9611463610973231, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 16403475722428553222}, 'section': 'Section::::Unified Rules of Mixed Martial Arts.\n'}]}]