Question: when did the us sign the nuclear non-proliferation treaty
[{'answer': '1968', 'provenance': [{'wikipedia_id': '22107', 'title': 'Treaty on the Non-Proliferation of Nuclear Weapons', 'start_paragraph_id': 1, 'start_character': 397, 'end_paragraph_id': 1, 'end_character': 401, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '22107', 'title': 'Treaty on the Non-Proliferation of Nuclear Weapons', 'start_paragraph_id': 19, 'start_character': 0, 'end_paragraph_id': 19, 'end_character': 339, 'bleu_score': 0.962600167205617, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 11221866929914761084}, 'section': 'Section::::Treaty structure.:First pillar: Non-proliferation.\n'}]}]