Question: when was the theory of seafloor spreading proposed
[{'answer': '1960s', 'provenance': [{'wikipedia_id': '145698', 'title': 'Seafloor spreading', 'start_paragraph_id': 3, 'start_character': 316, 'end_paragraph_id': 3, 'end_character': 321, 'bleu_score': 1.0, 'section': 'Section::::History of study.\n'}]}, {'provenance': [{'wikipedia_id': '145698', 'title': 'Seafloor spreading', 'start_paragraph_id': 3, 'start_character': 0, 'end_paragraph_id': 3, 'end_character': 523, 'bleu_score': 0.9766041382465441, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17994026831727171702}, 'section': 'Section::::History of study.\n'}]}]