Question: why is the congo river not navigable for its entire course
[{'answer': 'the three major falls', 'provenance': [{'wikipedia_id': '44361', 'title': 'Congo River', 'start_paragraph_id': 45, 'start_character': 20, 'end_paragraph_id': 45, 'end_character': 41, 'bleu_score': 1.0, 'section': 'Section::::Economic importance.\n'}]}, {'provenance': [{'wikipedia_id': '44361', 'title': 'Congo River', 'start_paragraph_id': 45, 'start_character': 0, 'end_paragraph_id': 45, 'end_character': 311, 'bleu_score': 0.96188712955549, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 17671449903809312273}, 'section': 'Section::::Economic importance.\n'}]}]