Question: the current two party system in the united states is defined by which pair of parties
[{'answer': 'Republican and Democratic parties', 'provenance': [{'wikipedia_id': '31605', 'title': 'Two-party system', 'start_paragraph_id': 22, 'start_character': 976, 'end_paragraph_id': 22, 'end_character': 1009, 'bleu_score': 1.0, 'section': 'Section::::Examples.:United States.\n'}]}, {'provenance': [{'wikipedia_id': '31605', 'title': 'Two-party system', 'start_paragraph_id': 22, 'start_character': 0, 'end_paragraph_id': 22, 'end_character': 1265, 'bleu_score': 0.889842123199384, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 16438756571541443589}, 'section': 'Section::::Examples.:United States.\n'}]}]