Question: when did the double handed tennis backhand change
[{'answer': '1980s', 'provenance': [{'wikipedia_id': '2066950', 'title': 'Backhand', 'start_paragraph_id': 25, 'start_character': 311, 'end_paragraph_id': 25, 'end_character': 316, 'bleu_score': 1.0, 'section': 'Section::::Technique.\n'}]}, {'provenance': [{'wikipedia_id': '2066950', 'title': 'Backhand', 'start_paragraph_id': 27, 'start_character': 0, 'end_paragraph_id': 27, 'end_character': 1987, 'bleu_score': 0.979214200606021, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 12731489760393271972}, 'section': 'Section::::Technique.\n'}]}]