Question: when did the herald of free enterprise sink
[{'answer': '6 March 1987', 'provenance': [{'wikipedia_id': '523575', 'title': 'MS Herald of Free Enterprise', 'start_paragraph_id': 1, 'start_character': 150, 'end_paragraph_id': 1, 'end_character': 162, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '523575', 'title': 'MS Herald of Free Enterprise', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 195, 'bleu_score': 0.943604326170661, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15866000388659844576}, 'section': 'Section::::Abstract.'}]}]