Question: name the rivers that have made the plains of west bengal fertile
[{'answer': 'Ganges', 'provenance': [{'wikipedia_id': '4308757', 'title': 'Geography of West Bengal', 'start_paragraph_id': 6, 'start_character': 380, 'end_paragraph_id': 6, 'end_character': 386, 'bleu_score': 1.0, 'section': 'Section::::Political geography.\n'}]}, {'provenance': [{'wikipedia_id': '4308757', 'title': 'Geography of West Bengal', 'start_paragraph_id': 40, 'start_character': 0, 'end_paragraph_id': 40, 'end_character': 614, 'bleu_score': 0.9796927878356411, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 12628995776266809414}, 'section': 'Section::::Rivers.\n'}]}]