Question: what do the w and k stand for in call letters
[{'answer': 'the United States', 'provenance': [{'wikipedia_id': '355786', 'title': 'Call signs in North America', 'start_paragraph_id': 1, 'start_character': 356, 'end_paragraph_id': 1, 'end_character': 373, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '355786', 'title': 'Call signs in North America', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 445, 'bleu_score': 0.970362113628587, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 10044362189014320869}, 'section': 'Section::::Abstract.'}]}]