Question: what were the two most important commodities traded in early west african civilizations
[{'answer': 'salt', 'provenance': [{'wikipedia_id': '2435657', 'title': 'History of West Africa', 'start_paragraph_id': 2, 'start_character': 574, 'end_paragraph_id': 2, 'end_character': 578, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'meat and other foods', 'provenance': [{'wikipedia_id': '2435657', 'title': 'History of West Africa', 'start_paragraph_id': 32, 'start_character': 492, 'end_paragraph_id': 32, 'end_character': 512, 'bleu_score': 1.0, 'section': 'Section::::Cultural history.:Prehistory.\n'}]}, {'provenance': [{'wikipedia_id': '2435657', 'title': 'History of West Africa', 'start_paragraph_id': 32, 'start_character': 0, 'end_paragraph_id': 32, 'end_character': 642, 'bleu_score': 0.979484453366057, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 14033136672216384310}, 'section': 'Section::::Cultural history.:Prehistory.\n'}]}]