Question: which weapon was a primary cause of the u.s. entry into world war i
[{'answer': 'unrestricted submarine warfare', 'provenance': [{'wikipedia_id': '25162935', 'title': 'American entry into World War I', 'start_paragraph_id': 4, 'start_character': 293, 'end_paragraph_id': 4, 'end_character': 323, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]