Question: when did the secret service start protecting presidential candidates
[{'answer': 'In 1968', 'provenance': [{'wikipedia_id': '58221', 'title': 'United States Secret Service', 'start_paragraph_id': 47, 'start_character': 0, 'end_paragraph_id': 47, 'end_character': 7, 'bleu_score': 1.0, 'section': 'Section::::History.:20th century.:1960s.\n'}]}, {'provenance': [{'wikipedia_id': '58221', 'title': 'United States Secret Service', 'start_paragraph_id': 47, 'start_character': 0, 'end_paragraph_id': 47, 'end_character': 333, 'bleu_score': 0.9611463610973231, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1538704688758791810}, 'section': 'Section::::History.:20th century.:1960s.\n'}]}]