Question: when did the us start trade with china
[{'answer': 'February 28 , 1972', 'provenance': [{'wikipedia_id': '277880', 'title': 'China–United States relations', 'start_paragraph_id': 82, 'start_character': 20, 'end_paragraph_id': 82, 'end_character': 38, 'bleu_score': 1.0, 'section': "Section::::History.:The People's Republic of China and the United States.:Rapprochement.\n"}]}, {'provenance': [{'wikipedia_id': '277880', 'title': 'China–United States relations', 'start_paragraph_id': 82, 'start_character': 0, 'end_paragraph_id': 82, 'end_character': 1024, 'bleu_score': 0.9884053071617651, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 2250264739702251534}, 'section': "Section::::History.:The People's Republic of China and the United States.:Rapprochement.\n"}]}]