Question: why did the united states pursue a policy of containment
[{'answer': 'prevent the spread of communism', 'provenance': [{'wikipedia_id': '425279', 'title': 'Containment', 'start_paragraph_id': 1, 'start_character': 348, 'end_paragraph_id': 1, 'end_character': 379, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]