Question: when did the portuguese explore the gold coast of west africa
[{'answer': '15th century', 'provenance': [{'wikipedia_id': '2041178', 'title': 'European exploration of Africa', 'start_paragraph_id': 2, 'start_character': 83, 'end_paragraph_id': 2, 'end_character': 95, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '2041178', 'title': 'European exploration of Africa', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 787, 'bleu_score': 0.965400604802336, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 7116677622751719296}, 'section': 'Section::::Abstract.'}]}]