Question: why did many african immigrants come to the united states in the 1600s and 1700s
[{'answer': 'Africans were imported as slaves'}, {'provenance': [{'wikipedia_id': '14599622', 'title': 'History of immigration to the United States', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 716, 'bleu_score': 0.9387426391408241, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 8312159937881532597}, 'section': 'Section::::Abstract.'}]}]