Question: when did un troops approach the yalu river
[{'answer': 'During the Korean War', 'provenance': [{'wikipedia_id': '49559', 'title': 'Yalu River', 'start_paragraph_id': 17, 'start_character': 183, 'end_paragraph_id': 17, 'end_character': 204, 'bleu_score': 1.0, 'section': 'Section::::History.\n'}]}, {'provenance': [{'wikipedia_id': '49559', 'title': 'Yalu River', 'start_paragraph_id': 17, 'start_character': 0, 'end_paragraph_id': 17, 'end_character': 817, 'bleu_score': 0.913924794563801, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17842468544708759218}, 'section': 'Section::::History.\n'}]}]