Question: what were the two causes of the dust bowl
[{'answer': 'severe drought', 'provenance': [{'wikipedia_id': '59749', 'title': 'Dust Bowl', 'start_paragraph_id': 1, 'start_character': 154, 'end_paragraph_id': 1, 'end_character': 168, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'a failure to apply dryland farming methods to prevent wind erosion ( the Aeolian processes )'}, {'answer': 'a failure to apply dryland farming methods to prevent wind erosion'}, {'answer': 'failure to apply dryland farming methods to prevent wind erosion'}, {'provenance': [{'wikipedia_id': '59749', 'title': 'Dust Bowl', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 1006, 'bleu_score': 0.5666769387542301, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1324174396492623639}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '59749', 'title': 'Dust Bowl', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 0, 'bleu_score': 1.0, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 1324174396492623639}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '59749', 'title': 'Dust Bowl', 'section': 'Section::::Abstract.', 'start_paragraph_id': 1, 'end_paragraph_id': 1, 'meta': {'evidence_span': ['The Dust Bowl was a period of severe dust storms that greatly damaged the ecology and agriculture of the American and Canadian prairies during the 1930s; severe drought and a failure to apply dryland farming methods to prevent the aeolian processes (wind erosion) caused the phenomenon.', 'The Dust Bowl was a period of severe dust storms that greatly damaged the ecology and agriculture of the American and Canadian prairies during the 1930s; severe drought and a failure to apply dryland farming methods to prevent the aeolian processes (wind erosion) caused the phenomenon.', 'The Dust Bowl was a period of severe dust storms that greatly damaged the ecology and agriculture of the American and Canadian prairies during the 1930s; severe drought and a failure to apply dryland farming methods to prevent the aeolian processes (wind erosion) caused the phenomenon']}}]}, {'provenance': [{'wikipedia_id': '59749', 'title': 'Dust Bowl', 'section': 'Section::::Drought and dust storms.\n', 'start_paragraph_id': 12, 'end_paragraph_id': 12, 'meta': {'evidence_span': ['When severe drought struck the Great Plains region in the 1930s, it resulted in erosion and loss of topsoil because of farming practices at the time. The drought dried the topsoil and over time it became friable, reduced to a powdery consistency in some places.', 'When severe drought struck the Great Plains region in the 1930s, it resulted in erosion and loss of topsoil because of farming practices at the time.']}}]}]