Question: when was the zebra mussel introduced to the us
[{'answer': '1980s', 'provenance': [{'wikipedia_id': '488945', 'title': 'Zebra mussel', 'start_paragraph_id': 1, 'start_character': 308, 'end_paragraph_id': 1, 'end_character': 313, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}]