Question: when did us open switch to hard court
[{'answer': '1978', 'provenance': [{'wikipedia_id': '82729', 'title': 'US Open (tennis)', 'start_paragraph_id': 3, 'start_character': 214, 'end_paragraph_id': 3, 'end_character': 218, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]