Question: how are leaders of the two parties in congress chosen
[{'answer': 'Senate Democratic Caucus', 'provenance': [{'wikipedia_id': '46507', 'title': 'Party leaders of the United States Senate', 'start_paragraph_id': 1, 'start_character': 451, 'end_paragraph_id': 1, 'end_character': 475, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'Senate Republican Conference', 'provenance': [{'wikipedia_id': '46507', 'title': 'Party leaders of the United States Senate', 'start_paragraph_id': 1, 'start_character': 484, 'end_paragraph_id': 1, 'end_character': 512, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'elected to their positions in the Senate by their respective party caucuses , the Senate Democratic Caucus and the Senate Republican Conference'}, {'provenance': [{'wikipedia_id': '46507', 'title': 'Party leaders of the United States Senate', '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': 13448501893808516465}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '46507', 'title': 'Party leaders of the United States Senate', 'start_paragraph_id': 1, 'start_character': 0, 'end_paragraph_id': 1, 'end_character': 512, 'bleu_score': 0.9321992460304711, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 13448501893808516465}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '46507', 'title': 'Party leaders of the United States Senate', 'section': 'Section::::Abstract.', 'start_paragraph_id': 1, 'end_paragraph_id': 1, 'meta': {'evidence_span': ['They are elected to their positions in the Senate by the party caucuses: the Senate Democratic Caucus and the Senate Republican Conference.\rhighlight sentence(s) contai', 'The Senate Majority and Minority Leaders are two United States Senators and members of the party leadership of the United States Senate. These leaders serve as the chief Senate spokespeople for the political parties respectively holding the majority and the minority in the United States Senate, and manage and schedule the legislative and executive business of the Senate. They are elected to their positions in the Senate by the party caucuses: the Senate']}}]}]