Question: what did the north use to limit south's trade
[{'answer': 'The Union blockade', 'provenance': [{'wikipedia_id': '1984020', 'title': 'Union blockade', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 18, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '1984020', 'title': 'Union blockade', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 126, 'bleu_score': 0.910879922930628, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 15236038075566521237}, 'section': 'Section::::Abstract.'}]}]