Question: when did silver birch win the grand national
[{'answer': 'Saturday 14 April 2007', 'provenance': [{'wikipedia_id': '10662306', 'title': 'Silver Birch (horse)', 'start_paragraph_id': 1, 'start_character': 409, 'end_paragraph_id': 1, 'end_character': 431, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '10662306', 'title': 'Silver Birch (horse)', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 484, 'bleu_score': 0.9375874466695201, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 5890232861698191732}, 'section': 'Section::::Abstract.'}]}]